Remote Playback: create Controller/Observer to allow core/ to be notified of changes.

This will allow the video wake lock implementation in Blink to change its
behaviour depending on remote playback state and will allow the remote
playback overaly to be moved out of modules/media_controls/

Bug: 913668, 903793
Change-Id: I473254b8108e612c52bdb55f4bd880ab0441922f
Reviewed-on: https://chromium-review.googlesource.com/c/1374951
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616208}
6 files changed