Adjust RtcpReceiver to be testable with callbacks:

webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h

Index: webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h b/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h
index f8fb21130c74acdac21c1bebdd9a756c41bb61aa..ea26bcca67b02b9a909cc82dd43837e3ddfa2fd4 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h
+++ b/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.h
@@ -27,7 +27,7 @@
 
 namespace webrtc {
 
-class ModuleRtpRtcpImpl : public RtpRtcp {
+class ModuleRtpRtcpImpl : public RtpRtcp, public RTCPReceiver::SenderCallbacks {
  public:
   explicit ModuleRtpRtcpImpl(const RtpRtcp::Configuration& configuration);
 
@@ -205,7 +205,7 @@ class ModuleRtpRtcpImpl : public RtpRtcp {
 
   void SetTMMBRStatus(bool enable) override;
 
-  void SetTmmbn(std::vector<rtcp::TmmbItem> bounding_set);
+  void SetTmmbn(std::vector<rtcp::TmmbItem> bounding_set) override;
 
   uint16_t MaxPayloadLength() const override;
 
@@ -312,10 +312,11 @@ class ModuleRtpRtcpImpl : public RtpRtcp {
   StreamDataCountersCallback* GetSendChannelRtpStatisticsCallback()
       const override;
 
-  void OnReceivedNACK(const std::list<uint16_t>& nack_sequence_numbers);
-  void OnReceivedRtcpReportBlocks(const ReportBlockList& report_blocks);
-
-  void OnRequestSendReport();
+  void OnReceivedNACK(
+      const std::list<uint16_t>& nack_sequence_numbers) override;
+  void OnReceivedRtcpReportBlocks(
+      const ReportBlockList& report_blocks) override;
+  void OnRequestSendReport() override;
 
  protected:
   bool UpdateRTCPReceiveInformationTimers();