[DevTools] Route input events to the correct RenderWidgetHost

With OOPIFs, we route input events sent to main frame's target
to the correct RenderWidgetHost inside.

We might consider doing the same for non-main frames as well, or
instead prohibit input domain on them.

This does not touch gestures yet, those will follow in next patch.

Bug: 746266

Change-Id: I7ba87c6d227ea24a559ebb7b5ca6a3cf9764d0f0
Reviewed-on: https://chromium-review.googlesource.com/939540
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540419}
8 files changed