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Use a completion fence for the previous frame to avoid overstuffing GVR's
buffer queue and blocking on GVR frame Submit. This fixes the inconsistent
timing in this mode that was due to event handling being delayed during
the blocking wait. Based on initial testing this mode now seems to be
generally more efficient than the baseline ClientWait mode, so setting
it to default to get more data. This can be overridden via Finch
parameter if needed.

Bug: 760389
Change-Id: I8bdf84e7fa2659e26daa467d815dc9b862c99610
Reviewed-on: https://chromium-review.googlesource.com/905786Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534865}