Reduce forward declarations in core/dom

Source/core/dom/ShadowTreeStyleSheetCollection.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
@@ skipping 14 matching lines @@
  *
  */
 
 #ifndef ShadowTreeStyleSheetCollection_h
 #define ShadowTreeStyleSheetCollection_h
 
 #include "core/dom/TreeScopeStyleSheetCollection.h"
 
 namespace WebCore {
 
-class CSSStyleSheet;
 class ShadowRoot;
-class StyleSheet;
 class StyleSheetCollection;
 class StyleEngine;
 
 class ShadowTreeStyleSheetCollection FINAL : public TreeScopeStyleSheetCollection {
     WTF_MAKE_NONCOPYABLE(ShadowTreeStyleSheetCollection);
     WTF_MAKE_FAST_ALLOCATED_WILL_BE_REMOVED;
 public:
     explicit ShadowTreeStyleSheetCollection(ShadowRoot&);
 
     void updateActiveStyleSheets(StyleEngine*, StyleResolverUpdateMode);
 
     virtual void trace(Visitor* visitor) OVERRIDE
     {
         TreeScopeStyleSheetCollection::trace(visitor);
     }
 
 private:
     void collectStyleSheets(StyleEngine*, StyleSheetCollection&);
 };
 
 }
 
 #endif