Throttle frames in OffscreenCanvasFrameDispatcherImpl

Propagate gpu/compositor backpressure.  This prevents the browser from
hanging when OffscreenCanvas animations cannot render at 60fps because
they are GPU bound.

This change Makes OffscreenCanvas ignore the BeginFrame signal when
backpressure is detected.  It uses the call to
OffscreenCanvasFrameDispatcherImpl::DidReceiveCompositorFrameAck as a
signal to unblock the next animation frame.

BUG=708709

Review-Url: https://codereview.chromium.org/2817603004
Cr-Commit-Position: refs/heads/master@{#464177}
2 files changed