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}

pc/rtc_stats_integrationtest.cc

diff --git a/pc/rtc_stats_integrationtest.cc b/pc/rtc_stats_integrationtest.cc
index 6125137..a071786 100644
--- a/pc/rtc_stats_integrationtest.cc
+++ b/pc/rtc_stats_integrationtest.cc
@@ -799,6 +799,7 @@
                                         *inbound_stream.media_type == "audio");
     verifier.TestMemberIsOptionalIDReference(
         inbound_stream.remote_id, RTCRemoteOutboundRtpStreamStats::kType);
+    verifier.TestMemberIsDefined(inbound_stream.mid);
     verifier.TestMemberIsDefined(inbound_stream.track_identifier);
     if (inbound_stream.kind.is_defined() &&
         *inbound_stream.media_type == "video") {
@@ -926,6 +927,7 @@
       const RTCOutboundRTPStreamStats& outbound_stream) {
     RTCStatsVerifier verifier(report_.get(), &outbound_stream);
     VerifyRTCRTPStreamStats(outbound_stream, verifier);
+    verifier.TestMemberIsDefined(outbound_stream.mid);
     if (outbound_stream.kind.is_defined() &&
         *outbound_stream.media_type == "video") {
       verifier.TestMemberIsIDReference(outbound_stream.media_source_id,