Implement Custom Elements 'extends' option.

Source/core/dom/CustomElementException.h

 /*
  * 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:
  *
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 28 matching lines @@
 class ExceptionState;
 
 class CustomElementException {
 public:
     enum Reason {
         CannotRegisterFromExtension,
         ConstructorPropertyNotConfigurable,
         ContextDestroyedCheckingPrototype,
         ContextDestroyedCreatingCallbacks,
         ContextDestroyedRegisteringDefinition,
+        ExtendsIsInvalidName,
+        ExtendsIsCustomElementName,
         InvalidName,
-        NotYetImplemented,
-        PrototypeDoesNotExtendHTMLElementSVGElementNamespace,
-        PrototypeDoesNotExtendHTMLElementSVGElementPrototype,
         PrototypeInUse,
         PrototypeNotAnObject,
         TypeAlreadyRegistered
     };
 
     static void throwException(Reason, const AtomicString& type, ExceptionState&);
 
 private:
     CustomElementException();
 
     static String preamble(const AtomicString& type);
 };
 
 }
 
 #endif // CustomElementException_h