Make squashing work with subpixel layout

Source/core/rendering/compositing/CompositedLayerMapping.h

Index: Source/core/rendering/compositing/CompositedLayerMapping.h
diff --git a/Source/core/rendering/compositing/CompositedLayerMapping.h b/Source/core/rendering/compositing/CompositedLayerMapping.h
index a22b28bc15e2d5cbab73068095cf2a66f28927c1..c47e2c7c3605955829c467d622edd9fa83552547 100644
--- a/Source/core/rendering/compositing/CompositedLayerMapping.h
+++ b/Source/core/rendering/compositing/CompositedLayerMapping.h
@@ -49,10 +49,11 @@ struct GraphicsLayerPaintInfo {
     // an intermediate offset for a squashed RenderLayer, described with respect to the CompositedLayerMapping's
     // owning layer that would eventually have the m_squashingLayer. Once the shared GraphicsLayer's bounds are
     // known, then we can trivially convert this offset to m_squashingLayer's space.
-    IntSize offsetFromSquashingCLM;
+    LayoutSize offsetFromSquashingCLM;
 
     // Offset describing where this squashed RenderLayer paints into the shared GraphicsLayer backing.
     IntSize offsetFromRenderer;
+    LayoutSize subpixelAccumulation;
 
     GraphicsLayerPaintingPhase paintingPhase;
 
@@ -166,7 +167,7 @@ public:
     bool hasUnpositionedOverflowControlsLayers() const;
 
     // Returns true if the assignment actually changed the assigned squashing layer.
-    bool updateSquashingLayerAssignment(RenderLayer*, IntSize offsetFromSquashingCLM, size_t nextSquashedLayerIndex);
+    bool updateSquashingLayerAssignment(RenderLayer*, LayoutSize offsetFromSquashingCLM, size_t nextSquashedLayerIndex);
     void removeRenderLayerFromSquashingGraphicsLayer(const RenderLayer*);
 
     void finishAccumulatingSquashingLayers(size_t nextSquashedLayerIndex);