Teach render_host about a mojo WidgetInputHandler.

Implement a WidgetInputHandler as a legacy interface over Chrome IPC.
This will then allow us to dynamically swap it with a mojo backed one.

This change does not provide a mojo backed implementation of this
interface but just an implementation of the interface backed by Chrome IPC.
The new implementation will come in another CL.

BUG=722928
[email protected]

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: I03bb42524cec501a8b76f517629e0b3e3e76bba0
Reviewed-on: https://chromium-review.googlesource.com/541036
Commit-Queue: Dave Tapuska <[email protected]>
Reviewed-by: Daniel Cheng <[email protected]>
Reviewed-by: Antoine Labour <[email protected]>
Cr-Commit-Position: refs/heads/master@{#484707}
51 files changed