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This CL adds MediaNotificationProviderImpl as a
MediaNotificationServiceObserver and it will inform its own observer
when MediaNotificationListChange so that the Zenith button living in ash
can show/hide correspondingly.
This CL also modifies MediaNotificationService such that it can show
session from all profiles when needed.

Bug: 1121723

Change-Id: I778976a154dd00dda9f83b300dced364cadfb56f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2369653
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801894}