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Add the MediaSessionMetadataChanged event to the
observer that will be fired if the metadata
associated with the media session changes.

This will be used for the notification to display
metadata of the currently playing media.

BUG=875004

Change-Id: I17d4e2af8b1d3b5b6213ec2eaaf0c29eb58c5937
Reviewed-on: https://chromium-review.googlesource.com/c/1316659
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612749}