Add GetFocusedFrame to WebContentsAndroid

This adds the new GetFocusedFrame function to WebContentsAndroid. Related
internal CL is linked in the bug.

Change-Id: Ia7ec7eaf9b95cd618b15df57a1e61f23a1b55519
Bug: 951568
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574894
Commit-Queue: Mehran Mahmoudi <mahmoudi@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653693}
5 files changed