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I'll look into the paint issue with MirrorView separatgely,
but I think this is good change anyway because it can eliminate
activation animation when entering/exitting overview for performance.

Bug: 1083951
Test: manual see bug for repro step.

Change-Id: I02a9ed265b2ab36cf348b37937669777cfd76c41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523907
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825234}