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The previous attempt [1] added checks for whether the starting or ending
screenshot was taken. In the case of the starting screenshot missing, it
would signal the animation to remove some RootWindowDeskSwitchAnimators,
leading to a vector out of bounds.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2506356

Bug: 1148607
Test: added regression test
Change-Id: I7f14df877cb93619a0ca8a85f5d0ad434dda3ad2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536934
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828034}