Rename RtpRtcp::Configuration::media_send_ssrc to local_media_ssrc

The name media_send_ssrc makes less sense when used mostly for the
RtcpReceiver functionality.

The old member is still there and used as a fallback. That will be
cleaned away after downstream code is fixed.

Bug: webrtc:10774
Change-Id: I4ec18db76910f31dfe76bc9b137ffe89220d3fa8
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/149836
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28923}

modules/rtp_rtcp/source/rtp_rtcp_impl_unittest.cc

@@ -162,7 +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;
+    config.local_media_ssrc = kSenderSsrc;
 
     impl_.reset(new ModuleRtpRtcpImpl(config));
     impl_->SetRTCPStatus(RtcpMode::kCompound);