[Media Controller] Split observer out

At the moment mojom::MediaController uses the media
session observer. This splits it so it has its own
observer. This allows use to have a nullable info
parameter passed to MediaSessionInfo which can tell
observers that we are not bound to any media session.

BUG=892771

Change-Id: Ifcb1835e3901c82221c7daad1b0036caadd10288
Reviewed-on: https://chromium-review.googlesource.com/c/1422837
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626876}
16 files changed