Don't include ComputedStyle.h where not needed.

third_party/WebKit/Source/core/html/HTMLOptGroupElement.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, 2007, 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.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  *
  */
 
 #ifndef HTMLOptGroupElement_h
 #define HTMLOptGroupElement_h
 
 #include "core/CoreExport.h"
 #include "core/html/HTMLElement.h"
-#include "core/style/ComputedStyle.h"
 
 namespace blink {
 
+class ComputedStyle;
 class HTMLSelectElement;
 class HTMLDivElement;
 
 class CORE_EXPORT HTMLOptGroupElement final : public HTMLElement {
   DEFINE_WRAPPERTYPEINFO();
 
  public:
   static HTMLOptGroupElement* create(Document&);
 
   bool isDisabledFormControl() const override;
   String defaultToolTip() const override;
   HTMLSelectElement* ownerSelectElement() const;
 
   String groupLabelText() const;
   HTMLDivElement& optGroupLabelElement() const;
 
  private:
   explicit HTMLOptGroupElement(Document&);
+  ~HTMLOptGroupElement();
 
   bool supportsFocus() const override;
   void parseAttribute(const QualifiedName&,
                       const AtomicString&,
                       const AtomicString&) override;
   void accessKeyAction(bool sendMouseEvents) override;
   void didAddUserAgentShadowRoot(ShadowRoot&) override;
   void attachLayoutTree(const AttachContext& = AttachContext()) override;
   void detachLayoutTree(const AttachContext& = AttachContext()) override;
   bool matchesEnabledPseudoClass() const override;
   InsertionNotificationRequest insertedInto(ContainerNode*) override;
   void removedFrom(ContainerNode*) override;
 
   // <optgroup> might not have a layoutObject so we manually manage a cached
   // style.
   void updateNonComputedStyle();
   ComputedStyle* nonLayoutObjectComputedStyle() const override;
   PassRefPtr<ComputedStyle> customStyleForLayoutObject() override;
 
   void updateGroupLabel();
 
   RefPtr<ComputedStyle> m_style;
 };
 
 }  // namespace blink
 
 #endif  // HTMLOptGroupElement_h