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This CL adds MediaNotificationProvider and
MediaNotificationProviderObserver interface serve as a connection
between ash and browser. Which will be used to send
MediaNotificationListViews and media session info from browser to ash.

Bug: 1111420

Change-Id: I276ff5d10642450ab392096c0a6db7c00a1c1c41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315092Reviewed-by: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Tim Song <tengs@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Jazz Xu <jazzhsu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801489}