Change aggregated fraction loss to be calculated from the cumulative loss and extended sequence number diff between the current and the last report block of two get stats calls.

Previously it was derived from the fraction loss of the current report (which could be based on a received report block in between two get stats calls).

R=stefan@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/36399004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7928 4adac7df-926f-26a2-2b94-8c16560cd09d