stats: add transportId to codec stats

BUG=webrtc:12181

Change-Id: Ib8e38f19ef2ddcb98455356087781f146af8c6b5
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/193280
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32618}
diff --git a/api/stats/rtcstats_objects.h b/api/stats/rtcstats_objects.h
index 7d8f5f5..ee3d707 100644
--- a/api/stats/rtcstats_objects.h
+++ b/api/stats/rtcstats_objects.h
@@ -115,6 +115,7 @@
   RTCCodecStats(const RTCCodecStats& other);
   ~RTCCodecStats() override;
 
+  RTCStatsMember<std::string> transport_id;
   RTCStatsMember<uint32_t> payload_type;
   RTCStatsMember<std::string> mime_type;
   RTCStatsMember<uint32_t> clock_rate;