Prefer to forward-declare GraphicsContext in headers.

Source/platform/graphics/GraphicsLayer.cpp

 /*
  * Copyright (C) 2009 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 14 matching lines @@
 
 #include "config.h"
 
 #include "platform/graphics/GraphicsLayer.h"
 
 #include "SkImageFilter.h"
 #include "SkMatrix44.h"
 #include "platform/geometry/FloatRect.h"
 #include "platform/geometry/LayoutRect.h"
 #include "platform/graphics/GraphicsLayerFactory.h"
+#include "platform/graphics/Image.h"
 #include "platform/graphics/filters/SkiaImageFilterBuilder.h"
 #include "platform/graphics/skia/NativeImageSkia.h"
 #include "platform/scroll/ScrollableArea.h"
 #include "platform/text/TextStream.h"
 #include "public/platform/Platform.h"
 #include "public/platform/WebAnimation.h"
 #include "public/platform/WebCompositorSupport.h"
 #include "public/platform/WebFilterOperations.h"
 #include "public/platform/WebFloatPoint.h"
 #include "public/platform/WebFloatRect.h"
@@ skipping 1195 matching lines @@
 #ifndef NDEBUG
 void showGraphicsLayerTree(const WebCore::GraphicsLayer* layer)
 {
     if (!layer)
         return;
 
     String output = layer->layerTreeAsText(WebCore::LayerTreeIncludesDebugInfo);
     fprintf(stderr, "%s\n", output.utf8().data());
 }
 #endif