Add methods to register congestion controller observer after construction.

webrtc/modules/congestion_controller/include/send_side_congestion_controller.h

 /*
  *  Copyright (c) 2012 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 52 matching lines @@
                                PacketRouter* packet_router);
   SendSideCongestionController(const Clock* clock,
                                Observer* observer,
                                RtcEventLog* event_log,
                                std::unique_ptr<PacedSender> pacer);
   virtual ~SendSideCongestionController();
 
   void RegisterPacketFeedbackObserver(PacketFeedbackObserver* observer);
   void DeRegisterPacketFeedbackObserver(PacketFeedbackObserver* observer);
 
+  // Currently, there can be at most one observer.
+  void RegisterNetworkObserver(Observer* observer);
+  void DeRegisterNetworkObserver(Observer* observer);
+
   virtual void SetBweBitrates(int min_bitrate_bps,
                               int start_bitrate_bps,
                               int max_bitrate_bps);
   // Resets the BWE state. Note the first argument is the bitrate_bps.
   virtual void OnNetworkRouteChanged(const rtc::NetworkRoute& network_route,
                                      int bitrate_bps,
                                      int min_bitrate_bps,
                                      int max_bitrate_bps);
   virtual void SignalNetworkState(NetworkState state);
   virtual void SetTransportOverhead(size_t transport_overhead_bytes_per_packet);
@@ skipping 40 matching lines @@
 
  private:
   void MaybeTriggerOnNetworkChanged();
 
   bool IsSendQueueFull() const;
   bool IsNetworkDown() const;
   bool HasNetworkParametersToReportChanged(uint32_t bitrate_bps,
                                            uint8_t fraction_loss,
                                            int64_t rtt);
   const Clock* const clock_;
-  Observer* const observer_;
+  rtc::CriticalSection observer_lock_;
+  Observer* observer_ GUARDED_BY(observer_lock_);
   RtcEventLog* const event_log_;
   const std::unique_ptr<PacedSender> pacer_;
   const std::unique_ptr<BitrateController> bitrate_controller_;
   const std::unique_ptr<ProbeController> probe_controller_;
   const std::unique_ptr<RateLimiter> retransmission_rate_limiter_;
   TransportFeedbackAdapter transport_feedback_adapter_;
   rtc::CriticalSection network_state_lock_;
   uint32_t last_reported_bitrate_bps_ GUARDED_BY(network_state_lock_);
   uint8_t last_reported_fraction_loss_ GUARDED_BY(network_state_lock_);
   int64_t last_reported_rtt_ GUARDED_BY(network_state_lock_);
   NetworkState network_state_ GUARDED_BY(network_state_lock_);
   rtc::CriticalSection bwe_lock_;
   int min_bitrate_bps_ GUARDED_BY(bwe_lock_);
   std::unique_ptr<DelayBasedBwe> delay_based_bwe_ GUARDED_BY(bwe_lock_);
 
   rtc::ThreadChecker worker_thread_checker_;
 
   RTC_DISALLOW_IMPLICIT_CONSTRUCTORS(SendSideCongestionController);
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_CONGESTION_CONTROLLER_INCLUDE_SEND_SIDE_CONGESTION_CONTROLLER_H_