RTCIceCandidateStats.transportId added and collected.

BUG=webrtc:6756, chromium:632723, chromium:627816

Review-Url: https://codereview.webrtc.org/2597963002
Cr-Commit-Position: refs/heads/master@{#15875}
diff --git a/webrtc/api/stats/rtcstats_objects.h b/webrtc/api/stats/rtcstats_objects.h
index cad3bc5..a006b38 100644
--- a/webrtc/api/stats/rtcstats_objects.h
+++ b/webrtc/api/stats/rtcstats_objects.h
@@ -167,6 +167,7 @@
   RTCIceCandidateStats(const RTCIceCandidateStats& other);
   ~RTCIceCandidateStats() override;
 
+  RTCStatsMember<std::string> transport_id;
   RTCStatsMember<bool> is_remote;
   RTCStatsMember<std::string> ip;
   RTCStatsMember<int32_t> port;