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We only have once window observer function OnWindowDestroying, and it
only removes the observer unless it is the restore focus window. So
there is no point observing the other windows or storing their pointers.

Bug: none
Test: none
Change-Id: Ib66cb0c34c316b1d7ed8c30f3d110e8b32d7b2b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533857Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826847}