Refactor WebView to be a thin proxy class

Splits interface and implementation; all client calls are forwarded
to an abstract WebViewProvider interface, and the existing implementation
moved into the WebViewClassic implementor of this interface.

Originally taken from a snapshot from the development branch, by:
git diff HEAD 9a4c328a54cc05e5 | git apply
- but then rebased to keep up to date with master

Interdepends on webkit and Browser changes:
https://android-git.corp.google.com/g/158979
https://android-git.corp.google.com/g/167911

Change-Id: I91403f32654ff308934e95c832d17b292a7d9b2e