Revert of Clean away use of RtpAudioFeedback interface from RTP/RTCP receiver code.

webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h

Index: webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
diff --git a/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h b/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
index 9acef79f383ce13d2632643525ac55ed29773ddc..2367a68d89ec5fcc3c347820b0a04e9f3ff5c5e3 100644
--- a/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
+++ b/webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h
@@ -217,6 +217,16 @@
   virtual void OnIncomingCSRCChanged(const uint32_t CSRC, const bool added) = 0;
 };
 
+class RtpAudioFeedback {
+ public:
+  virtual void OnPlayTelephoneEvent(const uint8_t event,
+                                    const uint16_t lengthMs,
+                                    const uint8_t volume) = 0;
+
+ protected:
+  virtual ~RtpAudioFeedback() {}
+};
+
 class RtcpIntraFrameObserver {
  public:
   virtual void OnReceivedIntraFrameRequest(uint32_t ssrc) = 0;
@@ -347,6 +357,16 @@
   }
 };
 
+// Null object version of RtpAudioFeedback.
+class NullRtpAudioFeedback : public RtpAudioFeedback {
+ public:
+  virtual ~NullRtpAudioFeedback() {}
+
+  void OnPlayTelephoneEvent(const uint8_t event,
+                            const uint16_t lengthMs,
+                            const uint8_t volume) override {}
+};
+
 // Statistics about packet loss for a single directional connection. All values
 // are totals since the connection initiated.
 struct RtpPacketLossStats {