mash: Fix crash when clicking on the shelf overflow icon

ash/system/tray/tray_background_view.h

Index: ash/system/tray/tray_background_view.h
diff --git a/ash/system/tray/tray_background_view.h b/ash/system/tray/tray_background_view.h
index ecf8404441efcf8c0a47d6211f18df6964f96a74..2aecdd54226aa3485359d5586036a244e9b12515 100644
--- a/ash/system/tray/tray_background_view.h
+++ b/ash/system/tray/tray_background_view.h
@@ -68,6 +68,9 @@ class ASH_EXPORT TrayBackgroundView : public ActionableView,
   // Called after the tray has been added to the widget containing it.
   virtual void Initialize();
 
+  // Initializes animations for the bubble.
+  static void InitializeBubbleAnimations(views::Widget* bubble_widget);
+
   // views::View:
   void SetVisible(bool visible) override;
   const char* GetClassName() const override;
@@ -116,9 +119,6 @@ class ASH_EXPORT TrayBackgroundView : public ActionableView,
   void SetPaintsBackground(bool value,
                            BackgroundAnimatorChangeType change_type);
 
-  // Initializes animations for the bubble.
-  void InitializeBubbleAnimations(views::Widget* bubble_widget);
-
   // Returns the window hosting the bubble.
   aura::Window* GetBubbleWindowContainer() const;