GitLab

- Implemented ROAP on top of JSEP.
- Added a new signal after PortAllocatorSesison discoves all candidates for
  the channel.

TEST=try bots and apprtc.appspot.com
Review URL: https://chromiumcodereview.appspot.com/9727011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@127702 0039d316-1c4b-4281-b951-d872f2087c98