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..98d6f639a4da67ffa20b024359b882a7e294da91 100644
--- a/Source/core/rendering/RenderLayerCompositor.h
+++ b/Source/core/rendering/RenderLayerCompositor.h
@@ -232,6 +232,25 @@ private:
     void addToOverlapMap(OverlapMap&, RenderLayer*, IntRect& layerBounds, bool& boundsComputed);
     void addToOverlapMapRecursive(OverlapMap&, RenderLayer*, RenderLayer* ancestorLayer = 0);
 
+    struct SquashingState {
+        SquashingState()
+            : mostRecentMapping(0)
+            , hasMostRecentMapping(false)
+            , nextSquashedLayerIndex(0) { }
+
+        void updateSquashingStateForNewMapping(CompositedLayerMappingPtr, bool hasNewCompositedLayerMapping, IntSize newAbsToLocalOffset);
+
+        // 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;
+
+        // Counter that tracks what index the next RenderLayer would be if it gets squashed to the current squashing layer.
+        size_t nextSquashedLayerIndex;
+    };
+
     // 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*);
@@ -241,6 +260,7 @@ private:
 
     // Defines which RenderLayers will paint into which composited backings, by allocating and destroying CompositedLayerMappings as needed.
     void assignLayersToBackings(RenderLayer*, bool& layersChanged);
+    void assignLayersToBackingsInternal(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);