Implement pinch-zoom scaling for main-frame scrollbars and pinch-zoom overlay scrollbars.

cc/layer_impl.h

Index: cc/layer_impl.h
diff --git a/cc/layer_impl.h b/cc/layer_impl.h
index 5e0e32d02cde74620310bf8adb4275ad53fa1c07..d1b1680fb5e052d4fcdcd393d1d2d21f2a31cafa 100644
--- a/cc/layer_impl.h
+++ b/cc/layer_impl.h
@@ -298,6 +298,9 @@ public:
     void setVerticalScrollbarLayer(ScrollbarLayerImpl*);
     ScrollbarLayerImpl* verticalScrollbarLayer() { return m_verticalScrollbarLayer; }
 
+    void setHorizontalPinchZoomScrollbarLayer(ScrollbarLayerImpl*);
+    void setVerticalPinchZoomScrollbarLayer(ScrollbarLayerImpl*);
+
     gfx::Rect layerRectToContentRect(const gfx::RectF& layerRect) const;
 
     virtual skia::RefPtr<SkPicture> getPicture();
@@ -425,6 +428,12 @@ private:
     ScrollbarLayerImpl* m_horizontalScrollbarLayer;
     ScrollbarLayerImpl* m_verticalScrollbarLayer;
 
+    // Weak pointers to this layer's pinch-zoom scrollbars, if it has them.
+    // Either both or neither will be present. Updated during tree
+    // synchronization.
+    ScrollbarLayerImpl* m_horizontalPinchZoomScrollbarLayer;
+    ScrollbarLayerImpl* m_verticalPinchZoomScrollbarLayer;
+
     // Group of properties that need to be computed based on the layer tree
     // hierarchy before layers can be drawn.
     DrawProperties<LayerImpl, RenderSurfaceImpl> m_drawProperties;