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DWM composition is limited to primary monitor's refresh rate.  Prior to
GPU vsync we would use DWM composition rate to drive begin frames.

With GPU vsync we started using per monitor vblank to limit begin frames
in case primary monitor had higher refresh rate, but this might be
causing draw duration and omnibar latency regressions.  MS platform team
also suggested syncing to primary monitor only.  In any case the correct
approach would be to further limit begin frames based on event queries
from 2 frames back.

This CL makes the GPU vsync begin frames sync to primary monitor, and
also makes the vsync thread a singleton since it doesn't have per-window
state any more.

Bug: 953970
Change-Id: Id3d3f043cb847172b1d6ba4bd38b087ceb2d8631
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1711215Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679832}