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- FocusActivationStore uses ActivateWindow(nullptr) for deactivating
  to avoid an arbitrary window covering the active window before
  Restore().
- RootWindowController puts the moved windows under existing ones
  on display change.

Bug: 945754
Change-Id: I3cacfd3427b8c98c9ce9dd2da05977120c1c1d65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574824Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652678}