Land layer squashing behind a flag

Source/core/rendering/CompositedLayerMapping.h

Index: Source/core/rendering/CompositedLayerMapping.h
diff --git a/Source/core/rendering/CompositedLayerMapping.h b/Source/core/rendering/CompositedLayerMapping.h
index 8f1a5cdff2145e20f624c899588bf000b8f503f5..b0f85e44d0e9e48be192b684412d774033f834c9 100644
--- a/Source/core/rendering/CompositedLayerMapping.h
+++ b/Source/core/rendering/CompositedLayerMapping.h
@@ -52,6 +52,10 @@ struct GraphicsLayerPaintInfo {
 
     IntRect compositedBounds;
 
+    // A temporary offset used for squashing layers, when the origin of the
+    // squashing layer is not yet known.
+    IntSize offsetFromBackingRoot;
+
     IntSize offsetFromRenderer;
 
     GraphicsLayerPaintingPhase paintingPhase;
@@ -115,6 +119,8 @@ public:
     GraphicsLayer* parentForSublayers() const;
     GraphicsLayer* childForSuperlayers() const;
 
+    GraphicsLayer* squashingLayer() const { return m_squashingLayer.get(); }
+
     // Returns true for a composited layer that has no backing store of its own, so
     // paints into some ancestor layer.
     bool paintsIntoCompositedAncestor() const { return !(m_requiresOwnBackingStoreForAncestorReasons || m_requiresOwnBackingStoreForIntrinsicReasons); }
@@ -152,6 +158,9 @@ public:
     void positionOverflowControlsLayers(const IntSize& offsetFromRoot);
     bool hasUnpositionedOverflowControlsLayers() const;
 
+    void addRenderLayerToSquashingGraphicsLayer(RenderLayer*, IntSize offsetFromTargetBacking, size_t nextSquashedLayerIndex);
+    void finishAccumulatingSquashingLayers(size_t nextSquashedLayerIndex);
+
     // GraphicsLayerClient interface
     virtual void notifyAnimationStarted(const GraphicsLayer*, double wallClockTime, double monotonicTime) OVERRIDE;
 
@@ -210,6 +219,7 @@ private:
     bool updateScrollingLayers(bool scrollingLayers);
     void updateScrollParent(RenderLayer*);
     void updateClipParent(RenderLayer*);
+    bool updateSquashingLayers(bool needsSquashingLayers);
     void updateDrawsContent(bool isSimpleContainer);
     void registerScrollingLayers();
 
@@ -329,6 +339,10 @@ private:
 
     OwnPtr<WebAnimationProvider> m_animationProvider;
 
+    OwnPtr<GraphicsLayer> m_squashingContainmentLayer; // Only used if any squashed layers exist, to contain the squashed layers as siblings to the rest of the GraphicsLayer tree chunk.
+    OwnPtr<GraphicsLayer> m_squashingLayer; // Only used if any squashed layers exist, this is the backing that squashed layers paint into.
+    Vector<GraphicsLayerPaintInfo> m_squashedLayers;
+
     IntRect m_compositedBounds;
 
     bool m_artificiallyInflatedBounds; // bounds had to be made non-zero to make transform-origin work