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/api/rtpparameters.h b/api/rtpparameters.h
index d9ac1b6..12e0419 100644
--- a/api/rtpparameters.h
+++ b/api/rtpparameters.h
@@ -276,6 +276,11 @@
   static const char kPlayoutDelayUri[];
   static const int kPlayoutDelayDefaultId;
 
+  // Header extension for identifying media section within a transport.
+  // https://tools.ietf.org/html/draft-ietf-mmusic-sdp-bundle-negotiation-49#section-15
+  static const char kMidUri[];
+  static const int kMidDefaultId;
+
   // Encryption of Header Extensions, see RFC 6904 for details:
   // https://tools.ietf.org/html/rfc6904
   static const char kEncryptHeaderExtensionsUri[];