Make RootScroller set the outer viewport scroll layer in the compositor

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index fc802c8f391e818ab22895247b223b4c305a3ecc..c6d6715ac77645ba16caedd52fe8f70e853638fd 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -69,6 +69,7 @@ class RasterBufferProvider;
 class RenderPassDrawQuad;
 class RenderingStatsInstrumentation;
 class ResourcePool;
+class RootScrollerController;
 class ScrollElasticityHelper;
 class ScrollbarLayerImplBase;
 class SwapPromise;
@@ -501,6 +502,11 @@ class CC_EXPORT LayerTreeHostImpl
   TopControlsManager* top_controls_manager() {
     return top_controls_manager_.get();
   }
+
+  RootScrollerController* root_scroller_controller() {
+    return root_scroller_controller_.get();
+  }
+
   const GlobalStateThatImpactsTilePriority& global_tile_state() {
     return global_tile_state_;
   }
@@ -785,6 +791,7 @@ class CC_EXPORT LayerTreeHostImpl
   bool pinch_gesture_end_should_clear_scrolling_layer_;
 
   std::unique_ptr<TopControlsManager> top_controls_manager_;
+  std::unique_ptr<RootScrollerController> root_scroller_controller_;
 
   std::unique_ptr<PageScaleAnimation> page_scale_animation_;