Move InputMsg_SetFocus to mojo

This CL changes InputMsg_SetFocus so that it is implemented in the
RemoteFrame interface since it is only got through RenderFrameProxy.

It also introduces FakeRemoteFrame to use RemoteFrame on
render_frame_host_manager_unittest.cc.

Bug: 1039256
Change-Id: If7d1d2971fabf3c64777281b1bf5eedfa9fcf5c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1989714
Commit-Queue: Julie Kim <[email protected]>
Reviewed-by: Dave Tapuska <[email protected]>
Reviewed-by: Kentaro Hara <[email protected]>
Reviewed-by: Kinuko Yasuda <[email protected]>
Reviewed-by: Ken Rockot <[email protected]>
Cr-Commit-Position: refs/heads/master@{#732808}
19 files changed