GitLab

UnifiedSystemTray should not be hidden on the last notification removal,
which is different behavior from old NotificationTray.

TEST=manual
BUG=857336

Change-Id: I083d48455be19d591833a17c9514ca1dbfe6903b
Reviewed-on: https://chromium-review.googlesource.com/1124261Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572525}