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_sender_video_unittest.cc

@@ -147,7 +147,7 @@ class RtpSenderVideoTest : public ::testing::TestWithParam<bool> {
           config.outgoing_transport = &transport_;
           config.retransmission_rate_limiter = &retransmission_rate_limiter_;
           config.field_trials = &field_trials_;
-          config.media_send_ssrc = kSsrc;
+          config.local_media_ssrc = kSsrc;
           return config;
         }()),
         rtp_sender_video_(&fake_clock_, &rtp_sender_, nullptr, field_trials_) {