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In UnifiedSystemTray, notification list should have Clear All button at
the bottom of the list.

UX spec: http://shortn/_SbJoaT9xpn
Screenshot: http://screen/xdFEFjJTs5x

TEST=manual
BUG=851830

Change-Id: Ia08f4c773d6b155c53e3b189cc9cca743e12150d
Reviewed-on: https://chromium-review.googlesource.com/1096808Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567598}