Implement OffscreenCanvas Accelerated 2D commit()

This CL makes commit() on accelerated 2d context on OffscreenCanvas work.

BUG=563852
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2359723003
Cr-Commit-Position: refs/heads/master@{#420785}
8 files changed