Fix pixel snapping issues when layers become composited

Source/core/rendering/RenderLayer.h

Index: Source/core/rendering/RenderLayer.h
diff --git a/Source/core/rendering/RenderLayer.h b/Source/core/rendering/RenderLayer.h
index 1ac295c533118f8b0840214779c114ac8ff6a17c..1fbb0680882cee2b065f0f8282bf66be32048ccb 100644
--- a/Source/core/rendering/RenderLayer.h
+++ b/Source/core/rendering/RenderLayer.h
@@ -615,18 +615,22 @@ public:
     void paintOverlayScrollbars(GraphicsContext*, const LayoutRect& damageRect, PaintBehavior, RenderObject* paintingRoot = 0);
 
     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;
     };
 
     // This method figures out our layerBounds in coordinates relative to
@@ -682,7 +686,7 @@ public:
     bool overlapBoundsIncludeChildren() const { return hasFilter() && renderer()->style()->filter().hasFilterThatMovesPixels(); }
 
     // Can pass offsetFromRoot if known.
-    IntRect calculateLayerBounds(const RenderLayer* ancestorLayer, const LayoutPoint* offsetFromRoot = 0, CalculateLayerBoundsFlags = DefaultCalculateLayerBoundsFlags) const;
+    LayoutRect calculateLayerBounds(const RenderLayer* ancestorLayer, const LayoutPoint* offsetFromRoot = 0, CalculateLayerBoundsFlags = DefaultCalculateLayerBoundsFlags) const;
 
     // WARNING: This method returns the offset for the parent as this is what updateLayerPositions expects.
     LayoutPoint computeOffsetFromRoot(bool& hasLayerOffset) const;
@@ -948,7 +952,8 @@ private:
 
     void collectFragments(LayerFragments&, const RenderLayer* rootLayer, RenderRegion*, const LayoutRect& dirtyRect,
         ClipRectsType, OverlayScrollbarSizeRelevancy inOverlayScrollbarSizeRelevancy = IgnoreOverlayScrollbarSize,
-        ShouldRespectOverflowClip = RespectOverflowClip, const LayoutPoint* offsetFromRoot = 0, const LayoutRect* layerBoundingBox = 0);
+        ShouldRespectOverflowClip = RespectOverflowClip, const LayoutPoint* offsetFromRoot = 0,
+        const LayoutSize& subPixelAccumulation = LayoutSize(), const LayoutRect* layerBoundingBox = 0);
     void updatePaintingInfoForFragments(LayerFragments&, const LayerPaintingInfo&, PaintLayerFlags, bool shouldPaintContent, const LayoutPoint* offsetFromRoot);
     void paintBackgroundForFragments(const LayerFragments&, GraphicsContext*, GraphicsContext* transparencyLayerContext,
         const LayoutRect& transparencyPaintDirtyRect, bool haveTransparency, const LayerPaintingInfo&, PaintBehavior, RenderObject* paintingRootForRenderer);