Add API for transmission smotthening.

BUG=818
TEST=Only API tests added now.

Review URL: https://webrtc-codereview.appspot.com/787009

git-svn-id: http://webrtc.googlecode.com/svn/trunk@2756 4adac7df-926f-26a2-2b94-8c16560cd09d
6 files changed