Land layer squashing behind a flag

Source/core/rendering/RenderLayerCompositor.h

Index: Source/core/rendering/RenderLayerCompositor.h
diff --git a/Source/core/rendering/RenderLayerCompositor.h b/Source/core/rendering/RenderLayerCompositor.h
index dfc918551584adbc2024084ce630b4fe31a43998..644431b0985a06f0e18c071baaf1d550458c7226 100644
--- a/Source/core/rendering/RenderLayerCompositor.h
+++ b/Source/core/rendering/RenderLayerCompositor.h
@@ -232,6 +232,19 @@ private:
     void addToOverlapMap(OverlapMap&, RenderLayer*, IntRect& layerBounds, bool& boundsComputed);
     void addToOverlapMapRecursive(OverlapMap&, RenderLayer*, RenderLayer* ancestorLayer = 0);
 
+    struct SquashingState {
+        SquashingState()
+            : mostRecentMapping(0)
+            , hasMostRecentMapping(false) { }
+
+        // The most recent composited backing that the layer should squash onto if needed.
+        CompositedLayerMappingPtr mostRecentMapping;
+        bool hasMostRecentMapping;
+
+        // Offset from absolute coordinates of the target's owning layer to the backing's own local space.
+        IntSize absToLocalOffset;
+    };
+
     // Forces an update for all frames of frame tree recursively. Used only when the mainFrame compositor is ready to
     // finish all deferred work.
     static void finishCompositingUpdateForFrameTree(Frame*);
@@ -240,7 +253,7 @@ private:
     void computeCompositingRequirements(RenderLayer* ancestorLayer, RenderLayer*, OverlapMap*, struct CompositingRecursionData&, bool& descendantHas3DTransform, Vector<RenderLayer*>& unclippedDescendants);
 
     // Defines which RenderLayers will paint into which composited backings, by allocating and destroying CompositedLayerMappings as needed.
-    void assignLayersToBackings(RenderLayer*, bool& layersChanged);
+    void assignLayersToBackings(RenderLayer*, SquashingState&, bool& layersChanged);
 
     // Recurses down the tree, parenting descendant compositing layers and collecting an array of child layers for the current compositing layer.
     void rebuildCompositingLayerTree(RenderLayer*, Vector<GraphicsLayer*>& childGraphicsLayersOfEnclosingLayer, int depth);