Code inject via Content Script gets removed

Question

I am creating a Chrome Extension that would draw an image above tag showing Flash content. I have written the content script in my "contentscript.js" file which does nothing but simply inserts another script file "backgroundscript.js" into page's body via a script tag.

The code works on few of sites having flash videos and shows the image right above the Flash Player window but on other sites it does not show up. Also on few it shows up for a fraction of a second and then gets disappeared.

Below is my manifest file:

{
  "name": "My First Chrome Extension",
  "version": "1.0.0.0",
  "description": "Tag videos from websites with a single click!",
  "icons": {   "16":"application_16.png","48":"application_48.png","128":"application_128.png" },
  "homepage_url": "http://www.myurl.com",
  "page_action":{
                "default_icon":{
                "19":"application_19.png",
                "38":"application_38.png"
            },
            "default_title":"VDownloader browser plugin"

        },
  "content_scripts": [ { "js": [ "contentscript.js" ], "matches": [ "http://*/*" ], "run_at" : "document_end", "all_frames": true} ],
  "plugins": [ { "path": "myplugin.dll", "public": true } ],
  "manifest_version":2,
  "minimum_chrome_version":"17",
  "offline_enabled":true,
  "permissions":["tabs","http://*/*", "https://*/*","contextMenus"],
  "web_accessible_resources":["application.png","backgroundscript.js"]
}

Also I checked by inserting "debugger;" into my script and monitoring code execution via console; the script gets added at least once into the page body. Even in the cases where no image button is shown.

Is this some kind of anit-XSS control implemented on the the more advanced sites not showing my button?

I would really appreciate your help as I have been trying to get over this from past couple of weeks without success :(

EDIT:

Please see the content script below:

var s = document.createElement('script');
s.src = chrome.extension.getURL("backgroundscript.js");
s.onload = function() {
    this.parentNode.removeChild(this);
};
(document.head||document.documentElement).appendChild(s);

Target sites:

1. Does not show the button: http://www.metacafe.com
2. Shows the button for a moment: http://www.dailymotion.com
3. Shows the button correctly: http://www.myreviewalarm.com

Please take a look at backgroundscript.js content:

var vdDelayTimer;
var vdUpdateInterval;
var playerButtons;

embedPlugin();

function embedPlugin() {
    var embed = document.createElement('embed');
    embed.setAttribute('type', 'application/x-myplugin');
    embed.setAttribute('id', 'myplugin');
    embed.setAttribute('style',   'width:1px;height:1px;position:absolute;left:0px;top:0px;');
    if(document.body!=null)
{
    document.body.appendChild(embed);
}
    testPlugin();
}

function testPlugin() {
    var plugin = document.getElementById('vdPlugin');
    if (plugin != null) {
        playerButtons = [];
        vdDelayTimer = setTimeout("appendButtons()", 2000);
    }
    else {
        window.alert('Plugin does not exist!');
    }
}

function updateButtons() {

    for (i = 0; i < playerButtons.length; i += 2) {
        updateButtonLocation(playerButtons[i], playerButtons[i + 1]);
    }
}

function updateButtonLocation(player, button) {

    var playerX = getX(player);
    var playerY = getY(player);
    var buttonHeight = 38;
    var buttonWidth = 196;
    var buttonX = playerX + player.offsetWidth - buttonWidth - 12;
    var buttonY = playerY - buttonHeight + 1;
    button.setAttribute('style', 'background-image: url(http://myurl.com/img/browser-download-button.png); width:' + buttonWidth + 'px;height:' + buttonHeight + 'px;position:absolute;left:' + buttonX + 'px;top:' + buttonY + 'px; cursor:pointer; cursor:hand;');
}

function appendButtons() {
    debugger;
    var objectTags = document.getElementsByTagName('object');
    for (var i = 0; i < objectTags.length; i++) {
        var objectTag = objectTags[i];
        if ((objectTag.hasAttribute('classid') && objectTag.getAttribute('classid') == 'clsid:D27CDB6E-AE6D-11cf-96B8-444553540000')
|| (objectTag.hasAttribute('type') && objectTag.getAttribute('type') == 'application/x-shockwave-flash')) {
            if (objectTag.offsetWidth >= 320 && objectTag.offsetHeight >= 240)
                createButton(objectTag, getX(objectTag), getY(objectTag),  objectTag.offsetWidth);
        }
    }
    var embedTags = document.getElementsByTagName('embed');
    for (var i = 0; i < embedTags.length; i++) {
        try {
            var embedTag = embedTags[i];
            if (embedTag.hasAttribute('type') && embedTag.getAttribute('type') == 'application/x-shockwave-flash') {
                if (embedTag.offsetWidth >= 320 && embedTag.offsetHeight >= 240)
                    createButton(embedTag, getX(embedTag), getY(embedTag), embedTag.offsetWidth);
            }
        }
        catch (err) { }
    }
    var videoTags = document.getElementsByTagName('video');
    for (var i = 0; i < videoTags.length; i++) {
        try {
            var videoTag = videoTags[i];
            if (videoTag.hasAttribute('src')) {
                if (videoTag.offsetWidth >= 320 && videoTag.offsetHeight >= 240)
                    createButton(videoTag, getX(videoTag), getY(videoTag), videoTag.offsetWidth);
            }
        }
        catch (err) { }
    }
    vdUpdateInterval = setInterval("updateButtons();", 500);
}

function createButton(videoTag, playerX, playerY, playerWidth) {
    debugger;
    var buttonHeight = 38;
    var buttonWidth = 196;
    var buttonX = playerX + playerWidth - buttonWidth - 12;
    var buttonY = playerY - buttonHeight + 1;
    var vdPlugin = document.getElementById('vdPlugin');
    var strLocation = window.location.toString();
   // if (isSupported(strLocation)) {
        var downloadButton = document.createElement('img');
        downloadButton.setAttribute('title', 'Tag this video');
        downloadButton.setAttribute('src', 'http://myurl.com/img/browser-download-button.png');
        downloadButton.setAttribute('style', 'background-image: url(http://myurl.com/img/browser-download-button.png); width:' + buttonWidth + 'px;height:' + buttonHeight + 'px;position:absolute;left:' + buttonX + 'px;top:' + buttonY + 'px; cursor:pointer;cursor:hand;z-index:2147483647;');
        downloadButton.setAttribute('onclick', 'javascript:document.getElementById(\'vdPlugin\').downloadNow(window.location.href);');
        downloadButton.setAttribute('oncontextmenu', 'javascript:return false;');
        document.body.appendChild(downloadButton);
        playerButtons.push(videoTag, downloadButton);
   // }
}

function getY(oElement) {
    var iReturnValue = 0;
    while (oElement != null) {
        iReturnValue += oElement.offsetTop;
        oElement = oElement.offsetParent;
    }
    return iReturnValue;
}

function getX(oElement) {
    var iReturnValue = 0;
    while (oElement != null) {
        iReturnValue += oElement.offsetLeft;
        oElement = oElement.offsetParent;
    }
    return iReturnValue;
}


function isSupported(url) {
    var regLen = supportedSites.length;
    var regEx;
    var res = false;
    try {
        for (var i = 0; i < regLen && res == false; i++) {
            regEx = new RegExp(supportedSites[i], "i");
            res = regEx.test(url);
        }
    }
    catch (ex) {
    }
    return res;
}