rtcicecandidate: add relayProtocol

which is already defined in webrtc-stats
  https://w3c.github.io/webrtc-stats/#dom-rtcicecandidatestats-relayprotocol
for local candidates

webrtc.html

@@ -5976,6 +5976,7 @@ <h4>
   readonly attribute DOMString? relatedAddress;
   readonly attribute unsigned short? relatedPort;
   readonly attribute DOMString? usernameFragment;
+  readonly attribute DOMString? relayProtocol;
   RTCIceCandidateInit toJSON();
 };</pre>
             <section>
@@ -6278,6 +6279,17 @@ <h2>
                   This carries the <code class="ice">ufrag</code> as defined in
                   section 15.4 of [[RFC5245]].
                 </dd>
+                <dt>
+                  <dfn>relayProtocol</dfn> of type <span class=
+                  "idlMemberType">DOMString</span>
+                </dt>
+                <dd>
+                  <p>
+                    It is the protocol used by the endpoint to communicate with the TURN server. This
+                    is only present for local relay candidates. Valid values are <code>"udp"</code>,
+                    <code>"tcp"</code>, or <code>"tls"</code>.
+                  </p>
+                </dd>
               </dl>
             </section>
             <section>