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This CL updates the relationship between MediaNotificationItem and
MediaNotificationController (now MediaNotificationControllerImpl) such
that MediaNotificationItem is only responsible for deciding when to
show or hide a media notification, and isn't responsible for how that
media notification is displayed. This is done by creating a
MediaNotificationController interface that
MediaNotificationControllerImpl implements.

This CL also updates the relationship of MediaNotificationItem and
MediaNotificationView so that MediaNotificationView calls methods on
MediaNotificationItem directly instead of calling them through the
MediaNotificationController or the message_center::MessageCenter.

This is part of a refactoring of the media notification to make it
reusable outside of ash/media.

Bug: 959478
Change-Id: I77d3f0329f0e8ee32d4cbb79829a0fdf50ac8113
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1615497Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660873}