Wire up MID send value to the PeerConnection API

Bug: webrtc:4050
Change-Id: I522cf8621e2cb639f54be2402174befd23e4af59
Reviewed-on: https://webrtc-review.googlesource.com/60962
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22610}
diff --git a/media/base/mediachannel.h b/media/base/mediachannel.h
index 90fa40c..0b37e5d 100644
--- a/media/base/mediachannel.h
+++ b/media/base/mediachannel.h
@@ -615,11 +615,15 @@
     ost << "codecs: " << VectorToString(this->codecs) << ", ";
     ost << "extensions: " << VectorToString(this->extensions) << ", ";
     ost << "max_bandwidth_bps: " << max_bandwidth_bps << ", ";
+    ost << "mid: " << (mid.empty() ? "<not set>" : mid) << ", ";
     ost << "}";
     return ost.str();
   }
 
   int max_bandwidth_bps = -1;
+  // This is the value to be sent in the MID RTP header extension (if the header
+  // extension in included in the list of extensions).
+  std::string mid;
 };
 
 struct AudioSendParameters : RtpSendParameters<AudioCodec> {