Fix pixel snapping issues when layers become composited

Source/core/rendering/RenderLayerClipper.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 36 matching lines @@
 
 #include "core/rendering/ClipRect.h"
 #include "core/rendering/RenderBox.h" // For OverlayScrollbarSizeRelevancy.
 
 namespace WebCore {
 
 class RenderLayer;
 class RenderRegion;
 
 struct ClipRectsContext {
-    ClipRectsContext(const RenderLayer* inRootLayer, RenderRegion* inRegion, ClipRectsType inClipRectsType, OverlayScrollbarSizeRelevancy inOverlayScrollbarSizeRelevancy = IgnoreOverlayScrollbarSize, ShouldRespectOverflowClip inRespectOverflowClip = RespectOverflowClip)
+    ClipRectsContext(const RenderLayer* inRootLayer, RenderRegion* inRegion, ClipRectsType inClipRectsType, OverlayScrollbarSizeRelevancy inOverlayScrollbarSizeRelevancy = IgnoreOverlayScrollbarSize, ShouldRespectOverflowClip inRespectOverflowClip = RespectOverflowClip, const LayoutSize& inSubPixelAccumulation = LayoutSize())
         : rootLayer(inRootLayer)
         , region(inRegion)
         , clipRectsType(inClipRectsType)
         , overlayScrollbarSizeRelevancy(inOverlayScrollbarSizeRelevancy)
         , respectOverflowClip(inRespectOverflowClip)
+        , subPixelAccumulation(inSubPixelAccumulation)
     { }
     const RenderLayer* rootLayer;
     RenderRegion* region;
     ClipRectsType clipRectsType;
     OverlayScrollbarSizeRelevancy overlayScrollbarSizeRelevancy;
     ShouldRespectOverflowClip respectOverflowClip;
+    const LayoutSize& subPixelAccumulation;
 };
 
 class RenderLayerClipper {
     WTF_MAKE_NONCOPYABLE(RenderLayerClipper);
 public:
     RenderLayerClipper(RenderLayerModelObject* renderer)
     : m_renderer(renderer)
     {
     }
 
@@ skipping 16 matching lines @@
 private:
     // FIXME: Could this be a RenderBox?
     RenderLayerModelObject* m_renderer;
 
     OwnPtr<ClipRectsCache> m_clipRectsCache;
 };
 
 } // namespace WebCore
 
 #endif // RenderLayerClipper_h