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/pc/rtcstats_integrationtest.cc b/webrtc/pc/rtcstats_integrationtest.cc
index 0765276..5775047 100644
--- a/webrtc/pc/rtcstats_integrationtest.cc
+++ b/webrtc/pc/rtcstats_integrationtest.cc
@@ -505,7 +505,7 @@
     verifier->TestMemberIsDefined(stream.is_remote);
     verifier->TestMemberIsDefined(stream.media_type);
     verifier->TestMemberIsIDReference(
-        stream.media_track_id, RTCMediaStreamTrackStats::kType);
+        stream.track_id, RTCMediaStreamTrackStats::kType);
     verifier->TestMemberIsIDReference(
         stream.transport_id, RTCTransportStats::kType);
     verifier->TestMemberIsIDReference(stream.codec_id, RTCCodecStats::kType);