mash: Preliminary support for shelf auto-hide

ash/shelf/shelf_widget.h

Index: ash/shelf/shelf_widget.h
diff --git a/ash/shelf/shelf_widget.h b/ash/shelf/shelf_widget.h
index 6013414bb7c1ea16044362482b10c6782888466c..ac6db558602d82cca4badc69fada1770ebcbd61d 100644
--- a/ash/shelf/shelf_widget.h
+++ b/ash/shelf/shelf_widget.h
@@ -101,6 +101,10 @@ class ASH_EXPORT ShelfWidget : public views::Widget,
  private:
   class DelegateView;
 
+  // views::Widget:
+  void OnMouseEvent(ui::MouseEvent* event) override;
+  void OnGestureEvent(ui::GestureEvent* event) override;
+
   // Owned by the shelf container's aura::Window.
   ShelfLayoutManager* shelf_layout_manager_;
   std::unique_ptr<Shelf> shelf_;