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}
8 files changed