Remove forward declarations from Source/core/dom/custom

Source/core/dom/custom/CustomElementProcessingStep.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 17 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef CustomElementProcessingStep_h
 #define CustomElementProcessingStep_h
 
 #include "wtf/Noncopyable.h"
 
 namespace blink {
 
-class Element;
-
 class CustomElementProcessingStep {
     WTF_MAKE_NONCOPYABLE(CustomElementProcessingStep);
 public:
     CustomElementProcessingStep() { }
 
     virtual ~CustomElementProcessingStep() { }
     virtual void dispatch(Element*) = 0;
     virtual bool isCreatedCallback() const { return false; }
 };
 
-}
+} // namespace blink
 
 #endif // CustomElementProcessingStep_h