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This cl handles the correct z-order of visible windows including
AlwaysOnTop windows in the new Overview animations.

Bug: 812497
Test: WindowSelectorTest.HandleAlwaysOnTopWindow
Change-Id: I29dfc6beaa4bf9a5b549a78d9291acd861f17764
Reviewed-on: https://chromium-review.googlesource.com/936453Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#540390}