Remove "top_down" flag in the message center code

ui/message_center/views/message_list_view.h

 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_MESSAGE_CENTER_VIEWS_MESSAGE_LIST_VIEW_H_
 #define UI_MESSAGE_CENTER_VIEWS_MESSAGE_LIST_VIEW_H_
 
 #include <list>
 #include <set>
 
@@ skipping 20 matching lines @@
 // repositioning.
 class MESSAGE_CENTER_EXPORT MessageListView
     : public views::View,
       public views::BoundsAnimatorObserver {
  public:
   class Observer {
    public:
     virtual void OnAllNotificationsCleared() = 0;
   };
 
-  explicit MessageListView(bool top_down);
+  MessageListView();
   ~MessageListView() override;
 
   void AddNotificationAt(MessageView* view, int i);
   void RemoveNotification(MessageView* view);
   void UpdateNotification(MessageView* view, const Notification& notification);
   void SetRepositionTarget(const gfx::Rect& target_rect);
   void ResetRepositionSession();
   void ClearAllClosableNotifications(const gfx::Rect& visible_scroll_rect);
 
   void AddObserver(Observer* observer);
@@ skipping 39 matching lines @@
   void AnimateClearingOneNotification();
 
   // List of MessageListView::Observer
   base::ObserverList<Observer> observers_;
 
   // The top position of the reposition target rectangle.
   int reposition_top_;
   int fixed_height_;
   bool has_deferred_task_;
   bool clear_all_started_;
-  bool top_down_;
   std::set<views::View*> adding_views_;
   std::set<views::View*> deleting_views_;
   std::set<views::View*> deleted_when_done_;
   std::list<views::View*> clearing_all_views_;
   views::BoundsAnimator animator_;
 
   // If true, the message loop will be quitted after the animation finishes.
   // This is just for tests and has no setter.
   bool quit_message_loop_after_animation_for_test_;
 
   base::WeakPtrFactory<MessageListView> weak_ptr_factory_;
   DISALLOW_COPY_AND_ASSIGN(MessageListView);
 };
 
 }  // namespace message_center
 
 #endif  // UI_MESSAGE_CENTER_VIEWS_MESSAGE_LIST_VIEW_H_