Make |extends| option in custom element behave correctly

Source/bindings/core/v8/CustomElementConstructorBuilder.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 77 matching lines @@
         if (!basePrototype.IsEmpty())
             m_prototype->SetPrototype(basePrototype);
     }
 
     if (tryCatch.HasCaught()) {
         tryCatch.ReThrow();
         return false;
     }
 
     AtomicString extends;
-    bool extendsProvidedAndNonNull = DictionaryHelper::get(*m_options, "extends", extends);
+    bool extendsProvidedAndNonNull = DictionaryHelper::get(*m_options, "extends", extends) && (extends != "null");
 
     if (tryCatch.HasCaught()) {
         tryCatch.ReThrow();
         return false;
     }
 
     if (!m_scriptState->perContextData()) {
         // FIXME: This should generate an InvalidContext exception at a later point.
         CustomElementException::throwException(CustomElementException::ContextDestroyedCheckingPrototype, type, exceptionState);
         tryCatch.ReThrow();
@@ skipping 185 matching lines @@
 
     ExceptionState exceptionState(ExceptionState::ConstructionContext, "CustomElement", info.Holder(), info.GetIsolate());
     CustomElementCallbackDispatcher::CallbackDeliveryScope deliveryScope;
     RefPtrWillBeRawPtr<Element> element = document->createElementNS(namespaceURI, tagName, maybeType->IsNull() ? nullAtom : type, exceptionState);
     if (exceptionState.throwIfNeeded())
         return;
     v8SetReturnValueFast(info, element.release(), document);
 }
 
 } // namespace blink