GitLab

Different from the current notification center, UnifiedSystemTray will
shrink its area when notifications are removed.

UX spec: http://shortn/_U7QSE9nhSk

TEST=manual
BUG=828752

Change-Id: I5c793d39f88aa81fb7a2fa90366e62fc5dc9af2d
Reviewed-on: https://chromium-review.googlesource.com/1011522Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550476}