gpu: Add flag for enabling asynchronous worker context.

Passing the --enable-gpu-async-worker-context causes the worker context
created by RenderThreadImpl to use a different GPU stream. This also
implies that the worker and compositor contexts cannot be in the same
share group. At the moment there is no extra synchronization between the
worker context and compositor context other than OrderingBarrier so
using this flag does not guarantee correct rendering.

R=piman@chromium.org,dcheng@chromium.org
BUG=525259
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/1949023005
Cr-Commit-Position: refs/heads/master@{#392645}
28 files changed