Implement Custom Elements 'extends' option.

LayoutTests/fast/dom/custom/resources/document-register-fuzz.js

 
 function setupObjectHooks(hooks)
 {
     // Wrapper for these object should be materialized before setting hooks.
     console.log;
-    document.register
+    document.register;
     HTMLSpanElement.prototype;
 
     Object.defineProperty(Object.prototype, "prototype", {
         get: function() { return hooks.prototypeGet(); },
         set: function(value) { return hooks.prototypeSet(value); }
     });
 
     Object.defineProperty(Object.prototype, "constructor", {
         get: function() { return hooks.constructorGet(); },
         set: function(value) { return hooks.constructorSet(value); }
     });
 
     return hooks;
 }
 
 function exerciseDocumentRegister()
 {
+    register('x-a', {});
+    register('x-b', {prototype: Object.create(HTMLElement.prototype)});
+}
+
+function register(name, options)
+{
     var myConstructor = null;
-    var myPrototype = Object.create(HTMLElement.prototype);
     try {
-        myConstructor = document.register("x-do-nothing", { prototype: myPrototype });
+        myConstructor = document.register(name, options);
     } catch (e) { }
 
     try {
         if (!myConstructor) {
             debug("Constructor object isn't created.");
             return;
         }
 
-        if (myConstructor.prototype != myPrototype) {
+        if (myConstructor.prototype != options.prototype) {
             console.log("FAIL: bad prototype");
             return;
          }
 
         var element = new myConstructor();
         if (!element)
             return;
         if (element.constructor != myConstructor) {
             console.log("FAIL: bad constructor");
             return;
          }
     } catch (e) { console.log(e); }
 }