OffscreenCanvasCompositorFrameSink uses CompositorFrameSinkSupport

This CL is a mechanical refactor to transition
OffscreenCanvasCompositorFrameSink to use CompositorFrameSinkSupport,
enabling it to share code today with Mus and Exo.

BUG=664547

Review-Url: https://codereview.chromium.org/2569923004
Cr-Commit-Position: refs/heads/master@{#438566}
3 files changed