There was an official release of turn by turn navigation for iOS and Android of the Mapbox SDK: https://www.mapbox.com/navigation-sdk/
They include things like:
- Automatic rerouting
- Open source components
- Turn-by-turn guidance
- Navigation map with traffic
- Drop-in navigation UI
- Walking, biking, and driving modes
On the other hand, and related with the WebViews availability of a turn by turn API, you could use any JavaScript option like GraphHopper: https://www.graphhopper.com/ I'm using it for a personal project and it has online and offline features. They 'only' provide the path and the instructions. And it's totally open source. You will have to build your navigation features on top.