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In order to look visually consistent with UnifiedSystemTray,
notification popup ordering should be inverted on Chrome OS. To
implement that, this CL adds MOVE_UP_FOR_INVERSE animation state to
MessagePopupCollection.

TEST=MessagePopupCollectionTest
BUG=876769

Change-Id: I0dcb5c20c8bc3e76f44b32c989b7c935d0e2f676
Reviewed-on: https://chromium-review.googlesource.com/c/1290450
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602259}