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Even if a client sends the post-rotation buffer and new orientation
at the same time during screen rotation, currently the orientation
lock is reset before the frame with the buffer isn't completely
processed.

This CL moves the reset logic from OnPostWidgetCommit() to
DidReceiveCompositorFrameAck();

BUG=b:118493249
TEST=Manually confirmed display rotation didn't cause black flashes
     for ARC apps.

Change-Id: I470bef456345b67cfee396c80157cc97e852b603
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2089562
Commit-Queue: Kazuki Takise <takise@chromium.org>
Auto-Submit: Kazuki Takise <takise@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748084}