Clean up forward declarations in Source/core

Source/core/html/HTMLFieldSetElement.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2010 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 10 matching lines @@
  *
  */
 
 #ifndef HTMLFieldSetElement_h
 #define HTMLFieldSetElement_h
 
 #include "core/html/HTMLFormControlElement.h"
 
 namespace blink {
 
-class FormAssociatedElement;
 class HTMLFormControlsCollection;
 
 class HTMLFieldSetElement final : public HTMLFormControlElement {
     DEFINE_WRAPPERTYPEINFO();
 public:
     static PassRefPtrWillBeRawPtr<HTMLFieldSetElement> create(Document&, HTMLFormElement*);
     virtual void trace(Visitor*) override;
     HTMLLegendElement* legend() const;
 
     PassRefPtrWillBeRawPtr<HTMLFormControlsCollection> elements();
@@ skipping 22 matching lines @@
     void refreshElementsIfNeeded() const;
 
     mutable FormAssociatedElement::List m_associatedElements;
     // When dom tree is modified, we have to refresh the m_associatedElements array.
     mutable uint64_t m_documentVersion;
 };
 
 } // namespace blink
 
 #endif // HTMLFieldSetElement_h