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Stack trace shows window destroying calling |animating_layer_| which is
the one of the windows associated with the destroyed window. My guess is
that it might be a dangling pointer, so when window is destroying, set
it to null.

I tried reproing with a test that destroys the windows at certain points
of the animation, but no luck :(.

Test: none
Bug: 1028815
Change-Id: I5baa2d3784836296c2afe063e6861aa43fb68e5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2111059
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752670}