Add viewport scrollbar class to support overlay scrollbars for pinch zoom virtual viewport.

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 57b7edfb6469183f62325015a774d26d8ff2cac9..7d72cf70a53c5daf5ae7bcf9857b5e4074ee1e7f 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -110,6 +110,9 @@ class CC_EXPORT LayerTreeImpl {
 
   void FindRootScrollLayer();
   void UpdateMaxScrollOffset();
+  void SetPinchViewportLayerIds(int page_scale_layer_id,
+                                int inner_viewport_scroll_layer_id,
+                                int outer_viewport_scroll_layer_id);
 
   SkColor background_color() const { return background_color_; }
   void set_background_color(SkColor color) { background_color_ = color; }
@@ -210,6 +213,10 @@ class CC_EXPORT LayerTreeImpl {
   SkColor background_color_;
   bool has_transparent_background_;
 
+  LayerImpl* page_scale_layer_;
+  LayerImpl* inner_viewport_scroll_layer_;
+  LayerImpl* outer_viewport_scroll_layer_;
+
   float page_scale_factor_;
   float page_scale_delta_;
   float sent_page_scale_delta_;