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Exiting overview mode may leave overview header widget still animating.
It is important not to attempt any window manipulation such as
re-stacking when the animation completes because the affected windows
may be gone by then.

BUG=646350
TEST=WindowSelectorTest.SafeToDestroyWindowDuringAnimation

Review-Url: https://codereview.chromium.org/2337383003
Cr-Commit-Position: refs/heads/master@{#418780}