Remove forward declarations from Source/core/dom/custom

Source/core/dom/custom/CustomElementCallbackInvocation.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 40 matching lines @@
         : m_callbacks(callbacks)
     {
     }
 
     CustomElementLifecycleCallbacks* callbacks() { return m_callbacks.get(); }
 
 private:
     RefPtr<CustomElementLifecycleCallbacks> m_callbacks;
 };
 
-}
+} // namespace blink
 
 #endif // CustomElementCallbackInvocation_h