Make RootScroller set the outer viewport scroll layer in the compositor

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 e07946f8dba77697fe0147b54dcb84355c34187f..37121a99bc79e6a92a93a74c48e22df175731f7e 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -207,6 +207,10 @@ class CC_EXPORT LayerTreeImpl {
                                 int inner_viewport_scroll_layer_id,
                                 int outer_viewport_scroll_layer_id);
   void ClearViewportLayers();
+
+  void SetRootScrollerLayerFromId(int root_scroller_layer_id);
+  LayerImpl* RootScrollerLayer() { return LayerById(root_scroller_layer_id_); }
+
   LayerImpl* OverscrollElasticityLayer() {
     return LayerById(overscroll_elasticity_layer_id_);
   }
@@ -514,6 +518,7 @@ class CC_EXPORT LayerTreeImpl {
   int page_scale_layer_id_;
   int inner_viewport_scroll_layer_id_;
   int outer_viewport_scroll_layer_id_;
+  int root_scroller_layer_id_;
 
   LayerSelection selection_;