Reland "Remove WaitSyncTokenCHROMIUM command"

This reverts commit c19106d109d4d36871c11bfb5a683d222583dd15.

Original change's description:
> 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-Commit-Position: refs/heads/master@{#599849}

Bug: 778753, 895984

TBR=dcheng@chromium.org,boliu@chromium.org,piman@chromium.org

Change-Id: I07943fdcbb131566718d51e78f134975ec53b62e
Reviewed-on: https://chromium-review.googlesource.com/c/1361870
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#615387}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 267428faed554af30b9f1193ce5447f0750621da
2 files changed