Implement Custom Elements 'extends' option.

Source/bindings/v8/V8PerContextData.cpp

 /*
  * Copyright (C) 2012 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 122 matching lines @@
             prototypeObject->SetPrototype(m_errorPrototype.newLocal(m_isolate));
     }
 
     m_constructorMap.set(type, UnsafePersistent<v8::Function>(m_isolate, function));
 
     return function;
 }
 
 v8::Local<v8::Object> V8PerContextData::prototypeForType(WrapperTypeInfo* type)
 {
-    return constructorForType(type)->Get(v8String("prototype", m_isolate)).As<v8::Object>();
+    v8::Local<v8::Object> constructor = constructorForType(type);
+    if (constructor.IsEmpty())
+        return v8::Local<v8::Object>();
+    return constructor->Get(v8String("prototype", m_isolate)).As<v8::Object>();
 }
 
 void V8PerContextData::addCustomElementBinding(CustomElementDefinition* definition, PassOwnPtr<CustomElementBinding> binding)
 {
     ASSERT(!m_customElementBindings->contains(definition));
     m_customElementBindings->add(definition, binding);
 }
 
 void V8PerContextData::clearCustomElementBinding(CustomElementDefinition* definition)
 {
@@ skipping 55 matching lines @@
     if (!data->IsString())
         return -1;
     v8::String::AsciiValue ascii(data);
     char* comma = strnstr(*ascii, ",", ascii.length());
     if (!comma)
         return -1;
     return atoi(comma + 1);
 }
 
 } // namespace WebCore