Ensures dereference of |video_frame_compositor_context_provider_| on proper thread.

This CL makes sure that the |video_frame_compositor_context_provider_|
and references to it get dereferenced to the thread it is bound to (in
this case, the |video_frame_compositor_task_runner_|

Bug: 898989
Change-Id: I98654bb5a6b29a43a384ce0308b4b97d575903a0
Reviewed-on: https://chromium-review.googlesource.com/c/1303178
Commit-Queue: CJ DiMeglio <lethalantidote@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605878}