Add a RemotingController which does the following:
1) Implements media::mojom::RemotingSource interface, and
   sends/Receives messages from/to a media::mojom::Remoter;
2) Monitors player events as a MediaPlayerObserver;
3) May trigger the switch of the media renderer between local
   playback and remoting.

BUG=643964

Review-Url: https://codereview.chromium.org/2389473002
Cr-Commit-Position: refs/heads/master@{#424791}