Remove GpuVideoAcceleratorFactories::SignalSyncToken

The only calller was GpuMemoryBufferVideoFramePool, but doesn't need to
any more as crbug.com/819914 was fixed.

Bug: 819914
Change-Id: Ife76f3a9ed7e287f94ea0db97f0c27a6179f4a3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1674408
Commit-Queue: Antoine Labour <piman@chromium.org>
Auto-Submit: Antoine Labour <piman@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672203}
8 files changed