[Media Controller] Add observer

Client using MediaController should be able to add
MediaSessionObserver's to observe the active media
session state.

The primary user here is the media notification which
needs to be notified of state and metadata updates
through media controller.

BUG=892771

Change-Id: I7aa343207d3eb2ffa713eb41272646c10e9e6784
Reviewed-on: https://chromium-review.googlesource.com/c/1298477
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602958}
8 files changed