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This CL adds SystemNotificationController that owns notification
controllers that are recently splitted from SystemTrayItems.
This CL also fixes initialization and destruction ordering of
SystemTrayModel and SystemNoticationController.

Other notification controllers might be added to
SystemNotificationController later.

TEST=ash_unittests
BUG=none

Change-Id: I13ac5e4f77ca7afbfe6f53753d207f29861448bf
Reviewed-on: https://chromium-review.googlesource.com/1025433Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553861}