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Spec wants a cross fade animation that is linear and 120ms. The target
bounds are often changing as well since the window is in a drag.

Modify current cross fade animation to take in different tween and
times. Since the animation happens while the window is in a drag and
the bounds are being updated, add a new animation observer. The new
created layer animates towards identity transform so changing the
bounds is ok. The old layer is a bit trickier to deal with so on a
animation tick, update the transform so that the old layer is always
aligned with the new layer.

Follow up will add metrics so we can have a perf test.

Test: manual
Bug: 1081986
Change-Id: I46882c0a11b7ea2c97b1c8802a9c65184b384c1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2204813Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770881}