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We need a way to let users know that notifications are hidden in lock
screen. Similar message was preaviously shown on NotificationTray in
lock screen.

Screenshot: http://screen/BNWeniKkuR5

TEST=manual
BUG=864964

Change-Id: If7fa3bf1e61b1bce77b355c907a4dfef254a8601
Reviewed-on: https://chromium-review.googlesource.com/1156203
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580103}