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According to UX spec, UnifiedSystemTray should keep the position of the
notification list from the bottom of the notification. Notifications
should be pushed up when UST is expanded, and the notification right
under the expand button should be the same one.

TEST=manual
BUG=856060

Change-Id: I0393dadca02a2e1d633a5a7e3fa165b7c4e3ba82
Reviewed-on: https://chromium-review.googlesource.com/1164807
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581172}