Put OffscreenCanvas.commit behind a flag

Bug: 563816
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: If09eece47845488c3da241b10bceb0e63aa06f7d
Reviewed-on: https://chromium-review.googlesource.com/1131221
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573867}
7 files changed