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Instead of a MediaDelegate returned by the ShellDelegate, we now have a
MediaController on the ash side, which receives a MediaClient from the
chrome side at startup.

This also makes minor changes to the interface to send information with
a notification instead of having ash try to read the information back
after notification.

BUG=647409

Review-Url: https://codereview.chromium.org/2563643003
Cr-Commit-Position: refs/heads/master@{#438046}