webrtc: allow codec transportId stats

BUG=webrtc:12181

Change-Id: Ifaa94e2ac7124a7c275093059c7f67790009aacc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2542382
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828090}

chrome/test/data/webrtc/peerconnection_getstats.js

@@ -303,6 +303,7 @@ addRTCStatsToWhitelist(
  * @private
  */
 let kRTCCodecStats = new RTCStats(null, {
+  transportId: 'string',
   payloadType: 'number',
   mimeType: 'string',
   // TODO(hbos): As soon as |codec| has been renamed |mimeType| in the webrtc