Refactoring temporal layers implementation and adding VideoCodecMode for easier control of codec settings.

Review URL: https://webrtc-codereview.appspot.com/1105007

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@3528 4adac7df-926f-26a2-2b94-8c16560cd09d
20 files changed