Implement OffscreenCanvas Unaccelerated 2d commit() on main thread

BUG=563852
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2360413002
Cr-Commit-Position: refs/heads/master@{#422231}
7 files changed