Add back method CongestionController::GetTransportFeedbackObserver.

This is a partial revert of https://codereview.webrtc.org/2725823002/,
to not break downstream applications.

BUG=webrtc:7058

TBR=stefan@webrtc.org

Review-Url: https://codereview.webrtc.org/2745613002
Cr-Commit-Position: refs/heads/master@{#17152}

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);