Fix pixel snapping issues when layers become composited

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 1df576f332d9a1e5a1db3ebe4c4eea1eff84fd98..4d5b48383395e4380b45e3a891edd6eb57437d16 100644
--- a/Source/core/rendering/RenderLayerBacking.h
+++ b/Source/core/rendering/RenderLayerBacking.h
@@ -123,8 +123,8 @@ public:
     void suspendAnimations(double time = 0);
     void resumeAnimations();
 
-    IntRect compositedBounds() const;
-    void setCompositedBounds(const IntRect&);
+    LayoutRect compositedBounds() const;
+    void setCompositedBounds(const LayoutRect&);
     void updateCompositedBounds();
 
     void updateAfterWidgetResize();
@@ -145,7 +145,7 @@ public:
     virtual void verifyNotPainting();
 #endif
 
-    IntRect contentsBox() const;
+    LayoutRect contentsBox() const;
     IntRect backgroundBox() const;
 
     // For informative purposes only.
@@ -189,7 +189,7 @@ private:
 
     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.
@@ -247,7 +247,8 @@ private:
 
     uint64_t m_scrollLayerID;
 
-    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;