Fix flickering in OffscreenCanvas WebGL

The fix prevents compositor frames from being pushed by
OffscreenCanvas whenever the push is unwarranted. Spurious pushes
cause flickering due to preserveDrawingBuffer=false behaviour.

BUG=863085

Change-Id: I24c8b4d27e6a07b41e8e0ae030d1ec626674b8d0
Reviewed-on: https://chromium-review.googlesource.com/1135520
Reviewed-by: Fernando Serboncini <[email protected]>
Commit-Queue: Justin Novosad <[email protected]>
Cr-Commit-Position: refs/heads/master@{#574768}
2 files changed