Split out RtcpCnameCallback from RtcpStatisticsCallback

Cname callback is used only on receive side, and statistics (soon)
only on the send side.

Bug: webrtc:10679
Change-Id: I122e9cafaea93cd0ba75dc955a652d9d4bddc379
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/147867
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28767}

modules/rtp_rtcp/source/rtp_rtcp_impl.h

@@ -242,6 +242,8 @@ class ModuleRtpRtcpImpl : public RtpRtcp, public RTCPReceiver::ModuleRtpRtcp {
   void RegisterRtcpStatisticsCallback(
       RtcpStatisticsCallback* callback) override;
   RtcpStatisticsCallback* GetRtcpStatisticsCallback() override;
+  void RegisterRtcpCnameCallback(RtcpCnameCallback* callback) override;
+
   void SetReportBlockDataObserver(ReportBlockDataObserver* observer) override;
 
   bool SendFeedbackPacket(const rtcp::TransportFeedback& packet) override;