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This is a reland of fc7ae248

Original change's description:
> Ash Tray: Refactor slide out animation to re-use transform animation.
> 
> There are currently two code paths to slide out a notification in the message
> center, which this CL rationalize this logic to just use the transform-based
> animation.
> 
> Furthermore, to implement the spec'd clear all animation, each MessageView
> needs to be animated separately, which isn't currently possible.
> 
> Note: the clear all animation still uses the old Layout() based slide-out
> animation. This will be removed once the spec'd clear all is implemented.
> 
> TEST=verified existing behaviour with normal and ARC++ notifications,
>      updated unit test
> BUG=897915
> 
> Change-Id: I294f4fe326730cf40a447c23d551e7d33838a579
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1613919
> Commit-Queue: Tim Song <tengs@chromium.org>
> Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#661553}

Bug: 897915
Change-Id: Ice73af1f3982d74f6b9c3006235165c52a0f4c7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1621539Reviewed-by: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662266}