Halt OffscreenCanvas animations when page is hidden

This CL uses a (possibly) cross-thread asynchronous signal to notify
OffscreenCanvasFrameDispatcherImpl that it's associated placeholder
canvas has changed its visibility state.  This information is in turn
used to suspend animation in the OffscreenCanvas.


Bug: 715757
Change-Id: Ib41a76585a8a3fdc79ca43cf30b6229d3015807d
Reviewed-on: https://chromium-review.googlesource.com/523664
Commit-Queue: Justin Novosad <junov@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#477297}
10 files changed