Make RTCIceCandidatePair members required

In webrtc-pc this dictionary is only used as a return type of getSelectedCandidatePair(), which will include both local and remote. This is not a problem.

In https://w3c.github.io/webrtc-extensions/#rtcicetransport the dictionary is used as a non-optional argument, and Web IDL requires that dictionary arguments are optional if there are no required members. Fix this by making them required.

webrtc.html

@@ -12559,8 +12559,8 @@ <h3>
           </h3>
           <div>
             <pre class="idl">dictionary RTCIceCandidatePair {
-  RTCIceCandidate local;
-  RTCIceCandidate remote;
+  required RTCIceCandidate local;
+  required RTCIceCandidate remote;
 };</pre>
             <section>
               <h2>