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This is 2nd attempt to land: crrev.com/c/1304973
[2] are the difference from above CL.

*[2] Added null check in OnStartingAnimationComplete.
* Pause occusion tracker during overview animation
 [2] wait 5000ms when exiting overview before resume
       because a user may re-enter overview
 [2] wait 50ms when entering overview before resume
       because it can take 2 frame to finish draing the frame
       after animation observer is called.
* Activate the text filter after animation
* [2]Animate shelf after overview animation.
* [2] Improve blur animation:
 Use Compositor's AnimationObesrver to drive animation
 Progress only by 2 to reduce the frequency.
* Don't animate backdrop window during overview animation.
* Fix the bounds change issue in WindowGrid. It shouldn't change
the parent window's bounds.

  in both clamshell and tabletmode.
  The web contents should load after animations without backdrop animation.
  No gap at the bottom of the screen after exiting overview mode.

Bug: 898077, 897387
Test: no functional change. Manually tested visual change by entering/exiting overview mode
Change-Id: I0e606af2ad2f8871108e17e6147e51d0b68b5ee6
Reviewed-on: https://chromium-review.googlesource.com/c/1316828Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605114}