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

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 2c733fc7ec82a2d48d869c953595a1bdee3f97c7..eb7786acd03b337686a511f11ad6f7a0ce08a99d 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -179,6 +179,14 @@ class CC_EXPORT LayerTreeHost : NON_EXPORTED_BASE(public RateLimiterClient) {
   Layer* root_layer() { return root_layer_.get(); }
   const Layer* root_layer() const { return root_layer_.get(); }
 
+  void RegisterPinchViewportLayers(
+      scoped_refptr<Layer> inner_viewport_clip_layer,
+      scoped_refptr<Layer> page_scale_layer,
+      scoped_refptr<Layer> inner_viewport_scroll_layer,
+      scoped_refptr<Layer> outer_viewport_scroll_layer,
+      scoped_refptr<Layer> inner_viewport_horizontal_scrollbar_layer,
+      scoped_refptr<Layer> inner_viewport_vertical_scrollbar_layer);
+
   const LayerTreeSettings& settings() const { return settings_; }
 
   void SetDebugState(const LayerTreeDebugState& debug_state);
@@ -303,6 +311,12 @@ class CC_EXPORT LayerTreeHost : NON_EXPORTED_BASE(public RateLimiterClient) {
   int num_failed_recreate_attempts_;
 
   scoped_refptr<Layer> root_layer_;
+  scoped_refptr<Layer> inner_viewport_clip_layer_;
+  scoped_refptr<Layer> page_scale_layer_;
+  scoped_refptr<Layer> inner_viewport_scroll_layer_;
+  scoped_refptr<Layer> outer_viewport_scroll_layer_;
+  scoped_refptr<Layer> inner_viewport_horizontal_scrollbar_layer_;
+  scoped_refptr<Layer> inner_viewport_vertical_scrollbar_layer_;
   scoped_refptr<HeadsUpDisplayLayer> hud_layer_;
 
   scoped_ptr<PrioritizedResourceManager> contents_texture_manager_;