Add support for accelerated fixed root background

Source/core/rendering/RenderLayerBacking.h

Index: Source/core/rendering/RenderLayerBacking.h
diff --git a/Source/core/rendering/RenderLayerBacking.h b/Source/core/rendering/RenderLayerBacking.h
index e4efc64407a816e1404402dc5e8df703ba5bf3d7..f7df53fc08410b211f2eb4f7d53e83bac19d5481 100644
--- a/Source/core/rendering/RenderLayerBacking.h
+++ b/Source/core/rendering/RenderLayerBacking.h
@@ -85,8 +85,6 @@ public:
     bool hasAncestorClippingLayer() const { return m_ancestorClippingLayer != 0; }
     GraphicsLayer* ancestorClippingLayer() const { return m_ancestorClippingLayer.get(); }
 
-    GraphicsLayer* contentsContainmentLayer() const { return m_contentsContainmentLayer.get(); }
-
     bool hasContentsLayer() const { return m_foregroundLayer != 0; }
     GraphicsLayer* foregroundLayer() const { return m_foregroundLayer.get(); }
 
@@ -241,7 +239,6 @@ private:
     RenderLayer* m_owningLayer;
 
     OwnPtr<GraphicsLayer> m_ancestorClippingLayer; // Only used if we are clipped by an ancestor which is not a stacking context.
-    OwnPtr<GraphicsLayer> m_contentsContainmentLayer; // Only used if we have a background layer; takes the transform.
     OwnPtr<GraphicsLayer> m_graphicsLayer;
     OwnPtr<GraphicsLayer> m_foregroundLayer; // Only used in cases where we need to draw the foreground separately.
     OwnPtr<GraphicsLayer> m_backgroundLayer; // Only used in cases where we need to draw the background separately.