Implement Outbound/InboundRtpStreamStats.mid.

This is what allowed us to remove "transceiver" stats from the spec.

Bug: webrtc:14191
Change-Id: I687a2dd97de016832005cb4271f6e1a0e0560cd3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/266022
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/main@{#37247}
diff --git a/api/stats/rtcstats_objects.h b/api/stats/rtcstats_objects.h
index 5465d15..b2ebdd6 100644
--- a/api/stats/rtcstats_objects.h
+++ b/api/stats/rtcstats_objects.h
@@ -452,6 +452,7 @@
   // TODO(https://crbug.com/webrtc/14174): Implement trackIdentifier and kind.
 
   RTCStatsMember<std::string> track_identifier;
+  RTCStatsMember<std::string> mid;
   RTCStatsMember<std::string> remote_id;
   RTCStatsMember<uint32_t> packets_received;
   RTCStatsMember<uint64_t> fec_packets_received;
@@ -515,6 +516,7 @@
 
   RTCStatsMember<std::string> media_source_id;
   RTCStatsMember<std::string> remote_id;
+  RTCStatsMember<std::string> mid;
   RTCStatsMember<std::string> rid;
   RTCStatsMember<uint32_t> packets_sent;
   RTCStatsMember<uint64_t> retransmitted_packets_sent;