Fix #852: WebSocket has implemented extensions

Author: annevk

source

@@ -92024,8 +92024,7 @@ interface <dfn>WebSocket</dfn> : <span>EventTarget</span> {
   connection is established</i>, its value might change, as defined below.</p>
 
   <p class="note">The <code data-x="dom-WebSocket-extensions">extensions</code> attribute returns
-  the extensions selected by the server, if any. (Currently this will only ever be the empty
-  string.)</p>
+  the extensions selected by the server, if any.</p>
 
   <p>The <dfn><code data-x="dom-WebSocket-protocol">protocol</code></dfn> attribute must initially
   return the empty string. After <i data-x="concept-websocket-established">the WebSocket connection