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/rtcstats_integrationtest.cc b/webrtc/api/rtcstats_integrationtest.cc
index eec8915..b9e8ac2 100644
--- a/webrtc/api/rtcstats_integrationtest.cc
+++ b/webrtc/api/rtcstats_integrationtest.cc
@@ -390,6 +390,8 @@
   bool VerifyRTCIceCandidateStats(
       const RTCIceCandidateStats& candidate) {
     RTCStatsVerifier verifier(report_, &candidate);
+    verifier.TestMemberIsIDReference(
+        candidate.transport_id, RTCTransportStats::kType);
     verifier.TestMemberIsDefined(candidate.is_remote);
     verifier.TestMemberIsDefined(candidate.ip);
     verifier.TestMemberIsNonNegative<int32_t>(candidate.port);