Fix pixel snapping issues when layers become composited

Source/core/rendering/CompositedLayerMapping.h

Index: Source/core/rendering/CompositedLayerMapping.h
diff --git a/Source/core/rendering/CompositedLayerMapping.h b/Source/core/rendering/CompositedLayerMapping.h
index 2a4cac9fe88416c1eb6fe6332bc8984be2b9b48b..187eb934abdad57bcb0151801362d193508e94c0 100644
--- a/Source/core/rendering/CompositedLayerMapping.h
+++ b/Source/core/rendering/CompositedLayerMapping.h
@@ -50,7 +50,7 @@ enum CompositingLayerType {
 struct GraphicsLayerPaintInfo {
     RenderLayer* renderLayer;
 
-    IntRect compositedBounds;
+    LayoutRect compositedBounds;
 
     IntSize offsetFromRenderer;
 
@@ -144,8 +144,8 @@ public:
     void animationPaused(double timeOffset, const String& name);
     void animationFinished(const String& name);
 
-    IntRect compositedBounds() const;
-    void setCompositedBounds(const IntRect&);
+    LayoutRect compositedBounds() const;
+    void setCompositedBounds(const LayoutRect&);
     void updateCompositedBounds();
 
     void updateAfterWidgetResize();
@@ -168,7 +168,7 @@ public:
     virtual void verifyNotPainting();
 #endif
 
-    IntRect contentsBox() const;
+    LayoutRect contentsBox() const;
     IntRect backgroundBox() const;
 
     // For informative purposes only.
@@ -188,6 +188,7 @@ public:
 
     virtual String debugName(const GraphicsLayer*) OVERRIDE;
 
+    LayoutSize subpixelAccumulation() const { return m_subpixelAccumulation; }
 private:
     void createPrimaryGraphicsLayer();
     void destroyGraphicsLayers();
@@ -213,11 +214,13 @@ private:
     void updateDrawsContent(bool isSimpleContainer);
     void registerScrollingLayers();
 
+    void adjustBoundsForSubPixelAccumulation(const RenderLayer* compositedAncestor, IntRect& localCompositingBounds, IntRect& relativeCompositingBounds, IntPoint& delta);
+
     void setBackgroundLayerPaintsFixedRootBackground(bool);
 
     GraphicsLayerPaintingPhase paintingPhaseForPrimaryLayer() const;
 
-    IntSize contentOffsetInCompostingLayer() const;
+    LayoutSize contentOffsetInCompostingLayer() const;
     // Result is transform origin in pixels.
     FloatPoint3D computeTransformOrigin(const IntRect& borderBox) const;
     // Result is perspective origin in pixels.
@@ -328,7 +331,8 @@ private:
 
     OwnPtr<WebAnimationProvider> m_animationProvider;
 
-    IntRect m_compositedBounds;
+    LayoutRect m_compositedBounds;
+    LayoutSize m_subpixelAccumulation; // The accumulated subpixel offset of the compositedBounds compared to absolute coordinates.
 
     bool m_artificiallyInflatedBounds; // bounds had to be made non-zero to make transform-origin work
     bool m_boundsConstrainedByClipping;