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This commit prepares ozone_demo to use GpuFences for overlay planes,
when support for them lands in any of the Ozone backends. The
--use-gpu-fences ozone_demo flag controls whether to enable the use of
fences or use the existing glFinish mechanism.

Bug: 828393
Test: Run ozone_demo on Ozone-DRM
Change-Id: I797e9445479ff4a9768cb89033d1dd6fd826b5ae
Reviewed-on: https://chromium-review.googlesource.com/1021693
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559412}