Let Call register ReceiveSideCongestionController as CallStatsObserver.

webrtc/modules/congestion_controller/include/receive_side_congestion_controller.h

 /*
  *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 28 matching lines @@
 
   virtual void OnReceivedPacket(int64_t arrival_time_ms,
                                 size_t payload_size,
                                 const RTPHeader& header);
 
   // TODO(nisse): Delete these methods, design a more specific interface.
   virtual RemoteBitrateEstimator* GetRemoteBitrateEstimator(bool send_side_bwe);
   virtual const RemoteBitrateEstimator* GetRemoteBitrateEstimator(
       bool send_side_bwe) const;
 
-  // TODO(nisse): Called by CongestionController, but not otherwise wired up.
   // Implements CallStatsObserver.
   void OnRttUpdate(int64_t avg_rtt_ms, int64_t max_rtt_ms) override;
 
   // This is send bitrate, used to control the rate of feedback messages.
   void OnBitrateChanged(int bitrate_bps);
 
   // Implements Module.
   int64_t TimeUntilNextProcess() override;
   void Process() override;
 
@@ skipping 37 matching lines @@
     RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(WrappingBitrateEstimator);
   };
 
   WrappingBitrateEstimator remote_bitrate_estimator_;
   RemoteEstimatorProxy remote_estimator_proxy_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_CONGESTION_CONTROLLER_INCLUDE_RECEIVE_SIDE_CONGESTION_CONTROLLER_H_