Plumb PinchVirtualViewport layers into CC.

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 5211ac89d1bb2d445776c0d7612ed602e894ff76..ba91e119df1a95545d9cfd7f1a388923fc4f4ae4 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -201,6 +201,10 @@ class CC_EXPORT LayerTreeHost : NON_EXPORTED_BASE(public RateLimiterClient) {
   void SetRootLayer(scoped_refptr<Layer> root_layer);
   Layer* root_layer() { return root_layer_.get(); }
   const Layer* root_layer() const { return root_layer_.get(); }
+  void RegisterViewportLayers(
+      scoped_refptr<Layer> page_scale_layer,
+      scoped_refptr<Layer> inner_viewport_scroll_layer,
+      scoped_refptr<Layer> outer_viewport_scroll_layer);
 
   const LayerTreeSettings& settings() const { return settings_; }
 
@@ -419,6 +423,10 @@ class CC_EXPORT LayerTreeHost : NON_EXPORTED_BASE(public RateLimiterClient) {
   LCDTextMetrics lcd_text_metrics_;
   int tree_id_;
 
+  scoped_refptr<Layer> page_scale_layer_;
+  scoped_refptr<Layer> inner_viewport_scroll_layer_;
+  scoped_refptr<Layer> outer_viewport_scroll_layer_;
+
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHost);
 };