Pull parts of TopLevelDisplayClient into DisplayCompositor

This CL starts to expose the beginnings of the API DisplayCompositor
should have:

SubmitCompositorFrame
RequestCopyOfOutput

TODO:

Hittest

TopLevelDisplayClient becomes a very thin implementation of
cc::DisplayClient and starts to resemble OnscreenDisplayClient.

BUG=610937, 610935

Review-Url: https://codereview.chromium.org/1976663003
Cr-Commit-Position: refs/heads/master@{#393731}
8 files changed