Prefer to forward-declare GraphicsContext in headers.

Source/platform/scroll/ScrollbarThemeMacCommon.h

 /*
  * Copyright (C) 2008 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 13 matching lines @@
  */
 
 #ifndef ScrollbarThemeMacCommon_h
 #define ScrollbarThemeMacCommon_h
 
 #include "platform/mac/NSScrollerImpDetails.h"
 #include "platform/scroll/ScrollbarTheme.h"
 
 namespace WebCore {
 
+class Pattern;
+
 class PLATFORM_EXPORT ScrollbarThemeMacCommon : public ScrollbarTheme {
 public:
     virtual ~ScrollbarThemeMacCommon();
 
     virtual void registerScrollbar(ScrollbarThemeClient*) OVERRIDE;
     virtual void unregisterScrollbar(ScrollbarThemeClient*) OVERRIDE;
     void preferencesChanged(float initialButtonDelay, float autoscrollButtonDelay, NSScrollerStyle preferredScrollerStyle, bool redraw);
 
     virtual bool supportsControlTints() const OVERRIDE { return true; }
 
@@ skipping 17 matching lines @@
     virtual void updateButtonPlacement() { }
 
     void paintGivenTickmarks(GraphicsContext*, ScrollbarThemeClient*, const IntRect&, const Vector<IntRect>&);
 
     RefPtr<Pattern> m_overhangPattern;
 };
 
 }
 
 #endif // ScrollbarThemeMacCommon_h