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Supervised user notification was generated in TraySupervisedUser, which
is a SystemTrayItem.

We're going to get rid of SystemTrayItems as UnifiedSystemTray will
replace them, so we should generate the notification outside system
tray.

TEST=SupervisedNotificationControllerTest
BUG=none

Change-Id: Ifc774e83eb28e996f3f337040f31d736ab8e5f52
Reviewed-on: https://chromium-review.googlesource.com/1013117Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551122}