Introduce WebMediaSession

WebMediaSession is the API enabling web exposed MediaSession objects
to control platform implementations of media session related
functionality.

Implementation will begin with Android and other platforms will be
added later.

Re-land of https://codereview.chromium.org/1370453002/ with
https://code.google.com/p/chromium/issues/detail?id=546394 fixed.

BUG=497735, 546394

Review URL: https://codereview.chromium.org/1370453002

Cr-Commit-Position: refs/heads/master@{#355058}

Review URL: https://codereview.chromium.org/1415923003

Cr-Commit-Position: refs/heads/master@{#355766}
26 files changed