Better buffer size estimation in NetEq for redundant packets

TEST=passed_all_trybots
R=henrik.lundin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15579005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6260 4adac7df-926f-26a2-2b94-8c16560cd09d