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.cc

@@ -666,6 +666,10 @@ RtcpStatisticsCallback* ModuleRtpRtcpImpl::GetRtcpStatisticsCallback() {
   return rtcp_receiver_.GetRtcpStatisticsCallback();
 }
 
+void ModuleRtpRtcpImpl::RegisterRtcpCnameCallback(RtcpCnameCallback* callback) {
+  rtcp_receiver_.RegisterRtcpCnameCallback(callback);
+}
+
 void ModuleRtpRtcpImpl::SetReportBlockDataObserver(
     ReportBlockDataObserver* observer) {
   return rtcp_receiver_.SetReportBlockDataObserver(observer);