Split up (Jingle)Session from BaseSession.  This is part of an ongoing effort to move Jingle-specific code out of WebRTC and into its own repository.

R=juberti@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/35529004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7922 4adac7df-926f-26a2-2b94-8c16560cd09d