Add back method CongestionController::GetTransportFeedbackObserver.

webrtc/modules/congestion_controller/include/congestion_controller.h

Index: webrtc/modules/congestion_controller/include/congestion_controller.h
diff --git a/webrtc/modules/congestion_controller/include/congestion_controller.h b/webrtc/modules/congestion_controller/include/congestion_controller.h
index 34547aa366eacf63cae7873c0a9c6129da0fa637..b8ba3b5a2cb33e672801c0eef152192d752c4067 100644
--- a/webrtc/modules/congestion_controller/include/congestion_controller.h
+++ b/webrtc/modules/congestion_controller/include/congestion_controller.h
@@ -94,6 +94,9 @@ class CongestionController : public CallStatsObserver,
   // TODO(nisse): Delete this accessor function. The pacer should be
   // internal to the congestion controller.
   virtual PacedSender* pacer() { return pacer_.get(); }
+  virtual TransportFeedbackObserver* GetTransportFeedbackObserver() {
+    return this;
+  }
   RateLimiter* GetRetransmissionRateLimiter();
   void EnablePeriodicAlrProbing(bool enable);