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On entering overview, windows that are stacked above the snapped window
need to stacked underneath, otherwise when we scroll it will scroll
over it.

Also changed the aspect ratio maintaining codes to account for the
overview margin and overview header.

Test: manual, added test
Bug: 981188
Change-Id: I99c368d5ab50931a1eabe7bc70bf8a5b738c2bea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1747027
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686147}