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The issue was the window was created on startup, then OnGetSwitchStates
triggers TabletMode to start and create the screenshot since it detects
a window animating. But the window doesn't destroy the screenshot because
a |fps_counter_| is not detected on animation end because it is created
in on animation schedules which happened before entering tablet mode.

This CL aims to prevent this and similar bugs by not taking the screenshot
if a window is already animating, so the only observed animation is the
one triggered by entering tablet mode. This may have missed data or
performance in some cases, but will prevent similar bugs.

Test: ash_unittests TabletModeControllerScreenshotTest.EnterTabletModeWhileAnimating
Bug: 1035356
Change-Id: I601d08ef50f214dbd262e5eea6baed40e5596270
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1999241
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#732159}