Iframe/div MD scrollbars get idle when mouse leaves the nested elements.

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 0ecbc9635b946683ae3444c86525306c325dfd98..d8eef482cf799d38c60598ca92039c4b378ca684 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -438,9 +438,6 @@ class CC_EXPORT LayerTreeHostImpl
   LayerImpl* OuterViewportScrollLayer() const;
   LayerImpl* CurrentlyScrollingLayer() const;
 
-  int scroll_layer_id_when_mouse_over_scrollbar() const {
-    return scroll_layer_id_when_mouse_over_scrollbar_;
-  }
   bool scroll_affects_scroll_handler() const {
     return scroll_affects_scroll_handler_;
   }
@@ -670,8 +667,6 @@ class CC_EXPORT LayerTreeHostImpl
 
   void ClearCurrentlyScrollingLayer();
 
-  void HandleMouseOverScrollbar(LayerImpl* layer_impl);
-
   LayerImpl* FindScrollLayerForDeviceViewportPoint(
       const gfx::PointF& device_viewport_point,
       InputHandler::ScrollInputType type,
@@ -748,8 +743,7 @@ class CC_EXPORT LayerTreeHostImpl
   bool did_lock_scrolling_layer_;
   bool wheel_scrolling_;
   bool scroll_affects_scroll_handler_;
-  int scroll_layer_id_when_mouse_over_scrollbar_;
-  int captured_scrollbar_layer_id_;
+  int scroll_layer_id_mouse_currently_over_;
 
   std::vector<std::unique_ptr<SwapPromise>>
       swap_promises_for_main_thread_scroll_update_;