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This is a reland of 536b412d

Original patch crashed with ASAN bots because it was trying to access
variable that was part of destroyed object.

Original change's description:
> desks: Speculative fix for crash.
>
> 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}

Bug: 1148607
Change-Id: Iec8fa6e68c182e2447399e74b42c742423a0c48f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2542598
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828575}