Add average rtt to CallStatsObserver and an average rtt histogram.

webrtc/video_engine/call_stats.cc

Index: webrtc/video_engine/call_stats.cc
diff --git a/webrtc/video_engine/call_stats.cc b/webrtc/video_engine/call_stats.cc
index 6e51eebe5a9d562c9c3c1582e13dc95b56a766b4..92855e379fe533cf6d5d39f0d75bf8c51f7e0132 100644
--- a/webrtc/video_engine/call_stats.cc
+++ b/webrtc/video_engine/call_stats.cc
@@ -123,7 +123,7 @@ int32_t CallStats::Process() {
   if (max_rtt_ms_ > 0) {
     for (std::list<CallStatsObserver*>::iterator it = observers_.begin();
          it != observers_.end(); ++it) {
-      (*it)->OnRttUpdate(max_rtt_ms_);
+      (*it)->OnRttUpdate(avg_rtt_ms_, max_rtt_ms_);
     }
   }
   return 0;