Prefer to forward-declare GraphicsContext in headers.

Source/platform/scroll/ScrollbarThemeMacCommon.mm

 /*
  * Copyright (C) 2008, 2011 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 16 matching lines @@
 #include "RuntimeEnabledFeatures.h"
 #include "platform/scroll/ScrollbarThemeMacCommon.h"
 
 #include <Carbon/Carbon.h>
 #include "platform/PlatformMouseEvent.h"
 #include "platform/graphics/Gradient.h"
 #include "platform/graphics/GraphicsContext.h"
 #include "platform/graphics/GraphicsContextStateSaver.h"
 #include "platform/graphics/GraphicsLayer.h"
 #include "platform/graphics/ImageBuffer.h"
+#include "platform/graphics/Pattern.h"
 #include "platform/mac/ColorMac.h"
 #include "platform/mac/LocalCurrentGraphicsContext.h"
 #include "platform/mac/NSScrollerImpDetails.h"
 #include "platform/mac/ScrollAnimatorMac.h"
 #include "platform/scroll/ScrollbarThemeClient.h"
 #include "platform/scroll/ScrollbarThemeMacNonOverlayAPI.h"
 #include "platform/scroll/ScrollbarThemeMacOverlayAPI.h"
 #include "public/platform/WebThemeEngine.h"
 #include "public/platform/Platform.h"
 #include "public/platform/WebRect.h"
@@ skipping 14 matching lines @@
 namespace WebCore {
 
 typedef HashSet<ScrollbarThemeClient*> ScrollbarSet;
 
 static ScrollbarSet& scrollbarSet()
 {
     DEFINE_STATIC_LOCAL(ScrollbarSet, set, ());
     return set;
 }
 
-}
-
-namespace WebCore {
-
 static float gInitialButtonDelay = 0.5f;
 static float gAutoscrollButtonDelay = 0.05f;
 static NSScrollerStyle gPreferredScrollerStyle = NSScrollerStyleLegacy;
 
 ScrollbarTheme* ScrollbarTheme::nativeTheme()
 {
     static ScrollbarThemeMacCommon* theme = NULL;
     if (theme)
         return theme;
     if (ScrollbarThemeMacCommon::isOverlayAPIAvailable()) {
@@ skipping 273 matching lines @@
 // static
 bool ScrollbarThemeMacCommon::isOverlayAPIAvailable()
 {
     static bool apiAvailable =
         [NSClassFromString(@"NSScrollerImp") respondsToSelector:@selector(scrollerImpWithStyle:controlSize:horizontal:replacingScrollerImp:)]
         && [NSClassFromString(@"NSScrollerImpPair") instancesRespondToSelector:@selector(scrollerStyle)];
     return apiAvailable;
 }
 
 } // namespace WebCore