Move painting code from RenderBoxModelObject into BoxPainter.

Source/core/rendering/RenderBox.h

Index: Source/core/rendering/RenderBox.h
diff --git a/Source/core/rendering/RenderBox.h b/Source/core/rendering/RenderBox.h
index 73e2bc3f6d1a25b7fccd86504f4fb48f4ff522cd..913f0e9b2f5a2ab8f58425295a17b0f1d0fe7fe3 100644
--- a/Source/core/rendering/RenderBox.h
+++ b/Source/core/rendering/RenderBox.h
@@ -632,7 +632,8 @@ protected:
     virtual void updateFromStyle() OVERRIDE;
 
     // Returns false if it could not cheaply compute the extent (e.g. fixed background), in which case the returned rect may be incorrect.
-    bool getBackgroundPaintedExtent(LayoutRect&) const;
+    // FIXME: make this a const method once the RenderBox reference in BoxPainter is const.
+    bool getBackgroundPaintedExtent(LayoutRect&);
     virtual bool foregroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect, unsigned maxDepthToTest) const;
     virtual bool computeBackgroundIsKnownToBeObscured() OVERRIDE;