RTCRTPStreamStats.mediaTrackId renamed to trackId.

According to spec change:
https://github.com/w3c/webrtc-stats/pull/142

BUG=webrtc:7064, chromium:685655

Review-Url: https://codereview.webrtc.org/2619353007
Cr-Commit-Position: refs/heads/master@{#16326}
diff --git a/webrtc/api/stats/rtcstats_objects.h b/webrtc/api/stats/rtcstats_objects.h
index 760ec9e..c5277e7 100644
--- a/webrtc/api/stats/rtcstats_objects.h
+++ b/webrtc/api/stats/rtcstats_objects.h
@@ -312,7 +312,7 @@
   // crbug.com/657855, 657856
   RTCStatsMember<bool> is_remote;  // = false
   RTCStatsMember<std::string> media_type;
-  RTCStatsMember<std::string> media_track_id;
+  RTCStatsMember<std::string> track_id;
   RTCStatsMember<std::string> transport_id;
   RTCStatsMember<std::string> codec_id;
   // FIR and PLI counts are only defined for |media_type == "video"|.