Guard setSelected with ProposalPending slot.

This avoids a race condition where an application might cause
state to change from within an event handler.

index.html

@@ -746,6 +746,11 @@ <h3>
           Let |candidatePair:RTCIceCandidatePair| be the candidate pair which is being {{nominated}}.
         </p>
       </li>
+      <li>
+        <p>
+          Set |transport|.{{RTCIceTransport/[[ProposalPending]]}} to <code>true</code>.
+        </p>
+      </li>
       <li>
         <p>
           Let |accepted:boolean| be the result of [= fire an event | firing an event =] named
@@ -755,6 +760,11 @@ <h3>
           |candidatePair|.
         </p>
       </li>
+      <li>
+        <p>
+          Set |transport|.{{RTCIceTransport/[[ProposalPending]]}} to <code>false</code>.
+        </p>
+      </li>
       <li>
         <p>
           If |accepted| is <code>false</code>, instruct the [= ICE agent =] to not {{nominate}} |candidatePair|, and
@@ -820,6 +830,11 @@ <h3>
           Let |transport:RTCIceTransport| be the {{RTCIceTransport}} object associated with this candidate pair.
         </p>
       </li>
+      <li>
+        <p>
+          If |transport|.{{RTCIceTransport/[[ProposalPending]]}} is <code>true</code>, [= exception/throw =] an {{InvalidStateError}}.
+        </p>
+      </li>
       <li>
         <p>
           If <var>transport</var>.{{RTCIceTransport/[[IceRole]]}} is {{RTCIceRole/"controlled"}}, [= exception/throw =] a
@@ -904,6 +919,11 @@ <h3>
           <code>false</code> otherwise.
         </p>
       </li>
+      <li>
+        <p>
+          Set |transport|.{{RTCIceTransport/[[ProposalPending]]}} to <code>true</code>.
+        </p>
+      </li>
       <li>
         <p>
           Let |accepted:boolean| be the result of [= fire an event | firing an event =] named
@@ -913,6 +933,11 @@ <h3>
           of |candidatePair|.
         </p>
       </li>
+      <li>
+        <p>
+          Set |transport|.{{RTCIceTransport/[[ProposalPending]]}} to <code>false</code>.
+        </p>
+      </li>
       <li>
         <p>
           If |accepted| is <code>false</code>, instruct the [= ICE agent =] to not remove the candidate pair indicated by
@@ -927,11 +952,14 @@ <h3>
       </li>
     </ol>
     <p>
-      The {{RTCIceTransport}} object is extended by adding the following internal slot:
+      The {{RTCIceTransport}} object is extended by adding the following internal slots:
     </p>
     <ul>
       <li>
-        <dfn data-dfn-for="RTCIceTransport">[[\CandidatePairNominated]]</dfn> initialized to <var>false</var>.
+        <dfn data-dfn-for="RTCIceTransport">[[\CandidatePairNominated]]</dfn> initialized to <code>false</code>.
+      </li>
+      <li>
+        <dfn data-dfn-for="RTCIceTransport">[[\ProposalPending]]</dfn> initialized to <code>false</code>.
       </li>
     </ul>
     <pre class="idl">
@@ -996,7 +1024,7 @@ <h2>Methods</h2>
           <p>
             The {{setSelectedCandidatePair}} method attempts to change the selected candidate pair. If successful, packets
             will be
-            sent and received on the provided candidate pair. When this method is invoked, the [= user agent =] must run
+            sent and received on the provided candidate pair. When this method is invoked, the [= user agent =] MUST run
             the
             steps to [= change the selected candidate pair =].
           </p>