Add ability to set RTCP sender ssrc at construction time

Bug: webrtc:10774
Change-Id: Iaf5857e24359e9795434bcd0cdbe1658a2f9f5d3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/144632
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28506}

modules/rtp_rtcp/source/rtp_rtcp_impl_unittest.cc

@@ -162,6 +162,7 @@ class RtpRtcpModule : public RtcpPacketTypeCounterObserver {
     config.rtcp_packet_type_counter_observer = this;
     config.rtt_stats = &rtt_stats_;
     config.rtcp_report_interval_ms = rtcp_report_interval_ms_;
+    config.media_send_ssrc = kSenderSsrc;
 
     impl_.reset(new ModuleRtpRtcpImpl(config));
     impl_->SetRTCPStatus(RtcpMode::kCompound);