Collecting RTCIceCandidatePairStats.transport_id and improved unittests

webrtc/api/stats/rtcstats.h

Index: webrtc/api/stats/rtcstats.h
diff --git a/webrtc/api/stats/rtcstats.h b/webrtc/api/stats/rtcstats.h
index aea47c22ad855236160fc6ce88b534574b46aef3..b3afee069561f01e5a4e041cbd4650f460481080 100644
--- a/webrtc/api/stats/rtcstats.h
+++ b/webrtc/api/stats/rtcstats.h
@@ -67,7 +67,8 @@ class RTCStats {
   // |Members| always returns the same members in the same order.
   std::vector<const RTCStatsMemberInterface*> Members() const;
   // Checks if the two stats objects are of the same type and have the same
-  // member values. These operators are exposed for testing.
+  // member values. Timestamps are not compared. These operators are exposed for
+  // testing.
   bool operator==(const RTCStats& other) const;
   bool operator!=(const RTCStats& other) const;