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This CL adds an interface for the InputHandler implementation to
communicate with the compositor. This CL is a start that mostly just
imposes an interface on the existing structure  of these objects. Future
CLs will continue to refine this into an interface that makes sense for
the compositor to expose. In particular, there are some cases that
definitely shouldn't be part of the compositor interface that require an
"escape hatch" where ThreadedInputHandler currently uses a reference to
the LayerTreeHostImpl:

- cc::Viewport
- The compositor Animation system
- FrameTrackers and metrics
- BrowserControls
- Scrollbar Animations
- Interaction with the PropertyTrees

These will need additional thought and more meaningful changes so will
be done separately.

Bug: 915926
Change-Id: I4bc79478f387614712c0f74bcb3c1355a917f591
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359390Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801553}