Bug 1218356 - P0 - Add WebIDL for RTCDataChannelStats;r=smaug

Spec: https://w3c.github.io/webrtc-stats/#dom-rtcdatachannelstats

Note: We do not yet support transport stats, and transportId acts
as key back into the collection of all stats, so for now transportId
must not appear in this object (it has no object to point to).
See bug 1225723

Differential Revision: https://phabricator.services.mozilla.com/D57114

dom/webidl/RTCStatsReport.webidl

@@ -14,6 +14,7 @@ enum RTCStatsType {
   "remote-inbound-rtp",
   "remote-outbound-rtp",
   "csrc",
+  "data-channel",
   "session",
   "track",
   "transport",
@@ -91,6 +92,19 @@ dictionary RTCRTPContributingSourceStats : RTCStats {
   DOMString     inboundRtpStreamId;
 };
 
+dictionary RTCDataChannelStats : RTCStats {
+  DOMString           label;
+  DOMString           protocol;
+  long                dataChannelIdentifier;
+  // RTCTransportId is not yet implemented - Bug 1225723
+  // DOMString transportId;
+  RTCDataChannelState state;
+  unsigned long       messagesSent;
+  unsigned long long  bytesSent;
+  unsigned long       messagesReceived;
+  unsigned long long  bytesReceived;
+};
+
 enum RTCStatsIceCandidatePairState {
   "frozen",
   "waiting",
@@ -155,6 +169,7 @@ dictionary RTCStatsCollection {
   sequence<RTCIceCandidateStats>            trickledIceCandidateStats = [];
   sequence<DOMString>                       rawLocalCandidates = [];
   sequence<DOMString>                       rawRemoteCandidates = [];
+  sequence<RTCDataChannelStats>             dataChannelStats = [];
 };
 
 // A collection of RTCStats dictionaries, plus some other info. Used by