Refactor RegisterViewportLayers to use a struct instead of 6 values

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 7456229a49ea709f818c8ef4d16d39eae23e70fe..51622245b1ef7feacc27912658df618094697975 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -241,29 +241,32 @@ class CC_EXPORT LayerTreeHost : public NON_EXPORTED_BASE(SurfaceReferenceOwner),
   Layer* root_layer() { return root_layer_.get(); }
   const Layer* root_layer() const { return root_layer_.get(); }
 
-  void RegisterViewportLayers(
-      scoped_refptr<Layer> overscroll_elasticity_layer,
-      scoped_refptr<Layer> page_scale_layer,
-      scoped_refptr<Layer> inner_viewport_container_layer,
-      scoped_refptr<Layer> outer_viewport_container_layer,
-      scoped_refptr<Layer> inner_viewport_scroll_layer,
-      scoped_refptr<Layer> outer_viewport_scroll_layer);
-
+  struct CC_EXPORT ViewportLayers {
+    ViewportLayers();
+    ~ViewportLayers();
+    scoped_refptr<Layer> overscroll_elasticity;
+    scoped_refptr<Layer> page_scale;
+    scoped_refptr<Layer> inner_viewport_container;
+    scoped_refptr<Layer> outer_viewport_container;
+    scoped_refptr<Layer> inner_viewport_scroll;
+    scoped_refptr<Layer> outer_viewport_scroll;
+  };
+  void RegisterViewportLayers(const ViewportLayers& viewport_layers);
   Layer* overscroll_elasticity_layer() const {
-    return overscroll_elasticity_layer_.get();
+    return viewport_layers_.overscroll_elasticity.get();
   }
-  Layer* page_scale_layer() const { return page_scale_layer_.get(); }
+  Layer* page_scale_layer() const { return viewport_layers_.page_scale.get(); }
   Layer* inner_viewport_container_layer() const {
-    return inner_viewport_container_layer_.get();
+    return viewport_layers_.inner_viewport_container.get();
   }
   Layer* outer_viewport_container_layer() const {
-    return outer_viewport_container_layer_.get();
+    return viewport_layers_.outer_viewport_container.get();
   }
   Layer* inner_viewport_scroll_layer() const {
-    return inner_viewport_scroll_layer_.get();
+    return viewport_layers_.inner_viewport_scroll.get();
   }
   Layer* outer_viewport_scroll_layer() const {
-    return outer_viewport_scroll_layer_.get();
+    return viewport_layers_.outer_viewport_scroll.get();
   }
 
   void RegisterSelection(const LayerSelection& selection);
@@ -571,12 +574,7 @@ class CC_EXPORT LayerTreeHost : public NON_EXPORTED_BASE(SurfaceReferenceOwner),
 
   scoped_refptr<Layer> root_layer_;
 
-  scoped_refptr<Layer> overscroll_elasticity_layer_;
-  scoped_refptr<Layer> page_scale_layer_;
-  scoped_refptr<Layer> inner_viewport_container_layer_;
-  scoped_refptr<Layer> outer_viewport_container_layer_;
-  scoped_refptr<Layer> inner_viewport_scroll_layer_;
-  scoped_refptr<Layer> outer_viewport_scroll_layer_;
+  ViewportLayers viewport_layers_;
 
   float top_controls_height_ = 0.f;
   float top_controls_shown_ratio_ = 0.f;