RTC[In/Out]boundRTPStreamStats.mediaTrackId collected.

webrtc/api/rtcstats_integrationtest.cc

Index: webrtc/api/rtcstats_integrationtest.cc
diff --git a/webrtc/api/rtcstats_integrationtest.cc b/webrtc/api/rtcstats_integrationtest.cc
index 545f80165f1c7ee7c5a08cf9416cee60109295ee..69dc834a11777a71fc8ba59e3314fd6516d590c8 100644
--- a/webrtc/api/rtcstats_integrationtest.cc
+++ b/webrtc/api/rtcstats_integrationtest.cc
@@ -490,7 +490,8 @@ class RTCStatsReportVerifier {
     verifier->TestMemberIsUndefined(stream.associate_stats_id);
     verifier->TestMemberIsDefined(stream.is_remote);
     verifier->TestMemberIsDefined(stream.media_type);
-    verifier->TestMemberIsUndefined(stream.media_track_id);
+    verifier->TestMemberIsIDReference(
+        stream.media_track_id, RTCMediaStreamTrackStats::kType);
     verifier->TestMemberIsIDReference(
         stream.transport_id, RTCTransportStats::kType);
     verifier->TestMemberIsIDReference(stream.codec_id, RTCCodecStats::kType);