Add average rtt to CallStatsObserver and an average rtt histogram.

webrtc/modules/remote_bitrate_estimator/test/estimators/remb.cc

Index: webrtc/modules/remote_bitrate_estimator/test/estimators/remb.cc
diff --git a/webrtc/modules/remote_bitrate_estimator/test/estimators/remb.cc b/webrtc/modules/remote_bitrate_estimator/test/estimators/remb.cc
index f0d57ad9bd854f0e0a0bd985afdc4b33edd8b494..8ec2b728db521bb7510b182b42ae497a120e00c1 100644
--- a/webrtc/modules/remote_bitrate_estimator/test/estimators/remb.cc
+++ b/webrtc/modules/remote_bitrate_estimator/test/estimators/remb.cc
@@ -77,7 +77,7 @@ RembReceiver::RembReceiver(int flow_id, bool plot)
   ss << "Estimate_" << flow_id_ << "#1";
   estimate_log_prefix_ = ss.str();
   // Default RTT in RemoteRateControl is 200 ms ; 50 ms is more realistic.
-  estimator_->OnRttUpdate(50);
+  estimator_->OnRttUpdate(50, 50);
 }
 
 RembReceiver::~RembReceiver() {