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This is a revised version of original attempt crrev.comc/c/2166804.
Instead of creating a new layers, this clone and animate the window's layer,
but makes sure that the animating layer is at the bottom of other layers
within the window.

* It adds a invisible view with a layer
  at the bottom of the non_client_view (no frame w/o this, so we can exclude such case).
* Adds the animating layer as a child of this view, with
  "SetMasksToBounds(false)" so that it will not be clipped during animation.
* Once animation is done, this animating, old layer will be removed.

Bug: 1073685
Test: covered by unittests.
Change-Id: Ib5ae6a218d200394c54932e5b559217fcc7f89e1
Fixed: 2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2375686Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802386}