New interface uses two bitrates (max/min). The pace multiplier is also removed from the interface and instead utilized outside. Min bitrate will be filled with padding if there's not enough media to transmit. Also fixes a bug in minimum transmission bitrate that made it ignore REMBs. A regression test has been added to catch it. BUG=3014 R=mflodman@webrtc.org, stefan@webrtc.org Review URL: https://webrtc-codereview.appspot.com/10059004 git-svn-id: http://webrtc.googlecode.com/svn/trunk@5723 4adac7df-926f-26a2-2b94-8c16560cd09d