Use else block, e.g.
if (is_chrome) {
document.write("Chrome message");
} else {
document.write("Another browser");
}
If you want to get other specific browsers you can use JS to detect them the same way with elseif:
var is_chrome = navigator.userAgent.toLowerCase().indexOf('chrome') > -1;
// Opera 8.0+
var isOpera = (!!window.opr && !!opr.addons) || !!window.opera || navigator.userAgent.indexOf(' OPR/') >= 0;
// Firefox 1.0+
var isFirefox = typeof InstallTrigger !== 'undefined';
if (is_chrome) {
document.write("Chrome message");
} else if (isOpera) {
document.write("Opera browser");
} else if (isFirefox) {
document.write("Firefox browser");
} else {
document.write("Another browser");
}