Make MessageCenter a singleton object and build on Windows.

ash/system/web_notification/web_notification_tray.h

 // Copyright (c) 2012 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 ASH_SYSTEM_WEB_NOTIFICATION_WEB_NOTIFICATION_TRAY_H_
 #define ASH_SYSTEM_WEB_NOTIFICATION_WEB_NOTIFICATION_TRAY_H_
 
 #include "ash/ash_export.h"
 #include "ash/system/tray/tray_background_view.h"
 #include "ash/system/tray/tray_views.h"
@@ skipping 53 matching lines @@
   // Updates tray visibility login status of the system changes.
   void UpdateAfterLoginStatusChange(user::LoginStatus login_status);
 
   // Returns true if the message center bubble is visible.
   bool IsMessageCenterBubbleVisible() const;
 
   // Returns true if the mouse is inside the notification bubble.
   bool IsMouseInNotificationBubble() const;
 
   message_center::MessageCenter* message_center() {
-    return message_center_.get();
+    return message_center_;
   }
 
   // Overridden from TrayBackgroundView.
   virtual void SetShelfAlignment(ShelfAlignment alignment) OVERRIDE;
   virtual void AnchorUpdated() OVERRIDE;
   virtual string16 GetAccessibleNameForTray() OVERRIDE;
   virtual void HideBubbleWithView(
       const views::TrayBubbleView* bubble_view) OVERRIDE;
   virtual bool ClickedOutsideBubble() OVERRIDE;
 
@@ skipping 60 matching lines @@
   }
 
   message_center::QuietModeBubble* quiet_mode_bubble() const {
     return quiet_mode_bubble_.get();
   }
 
   // Testing accessors.
   message_center::MessageCenterBubble* GetMessageCenterBubbleForTest();
   message_center::MessagePopupBubble* GetPopupBubbleForTest();
 
-  scoped_ptr<message_center::MessageCenter> message_center_;
+  message_center::MessageCenter* message_center_;
   scoped_ptr<internal::WebNotificationBubbleWrapper> message_center_bubble_;
   scoped_ptr<internal::WebNotificationBubbleWrapper> popup_bubble_;
   scoped_ptr<message_center::QuietModeBubble> quiet_mode_bubble_;
   views::ImageButton* button_;
   bool show_message_center_on_unlock_;
 
   DISALLOW_COPY_AND_ASSIGN(WebNotificationTray);
 };
 
 }  // namespace ash
 
 #endif  // ASH_SYSTEM_WEB_NOTIFICATION_WEB_NOTIFICATION_TRAY_H_