Define RtpTransportControllerSendInterface.

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 b8ba3b5a2cb33e672801c0eef152192d752c4067..44d3d2111f9ba701cfad12fac3645d1adc6a197f 100644
--- a/webrtc/modules/congestion_controller/include/congestion_controller.h
+++ b/webrtc/modules/congestion_controller/include/congestion_controller.h
@@ -92,8 +92,11 @@ class CongestionController : public CallStatsObserver,
       bool send_side_bwe);
   virtual int64_t GetPacerQueuingDelayMs() const;
   // TODO(nisse): Delete this accessor function. The pacer should be
-  // internal to the congestion controller.
+  // internal to the congestion controller. Currently needed by Call,
+  // to register the pacer module on the right thread.
   virtual PacedSender* pacer() { return pacer_.get(); }
+  // TODO(nisse): Delete this method, as soon as downstream projects
+  // are updated.
   virtual TransportFeedbackObserver* GetTransportFeedbackObserver() {
     return this;
   }