GitLab

Allocation of GpuMemoryBuffers requires a round-trip to the browser
process and by moving this to worker threads it is no longer on
the critical path on the compositor.

The LockForWrite mechanism in the ResourceProvider has been
adjusted to properly support worker thread allocation of
resources.

BUG=418553

Review URL: https://codereview.chromium.org/645943002

Cr-Commit-Position: refs/heads/master@{#301433}