Remove WaitSyncTokenCHROMIUM command

After migrating InProcessCommandBuffer to use GPU scheduler, most tasks
are scheduled after their sync token dependencies are satisifed (see
crrev.com/c/1157874).

The one exception was the WaitSyncToken IPC used by ReturnFrontBuffer
for pepper, which specifies a sync token, and waits while handling the
message.

Change ReturnFrontBuffer to contain the sync token, and use it to
specify the dependency to the scheduler.

Remove WaitSyncTokenCHROMIUM command, since sync token dependencies are
specified as task metadata in all cases.

Make other cleanups such as removing unnecessary sync token tests, and
make sure the sync token code paths are not used where unsupported by
using NOTREACHED.

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: Ieee4f6e2427a59a4e0c4b3c983cb489741241272
Bug: 778753
Reviewed-on: https://chromium-review.googlesource.com/c/1168155
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#599849}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c19106d109d4d36871c11bfb5a683d222583dd15
2 files changed