Make MessageCenter a singleton object and build on Windows.

ash/system/web_notification/web_notification_tray.h

Index: ash/system/web_notification/web_notification_tray.h
diff --git a/ash/system/web_notification/web_notification_tray.h b/ash/system/web_notification/web_notification_tray.h
index ffc34d87ece579b6737272d600aaf8378eea6e31..34285df7e6b61657c09cea0bbfe3f1abd1986464 100644
--- a/ash/system/web_notification/web_notification_tray.h
+++ b/ash/system/web_notification/web_notification_tray.h
@@ -71,7 +71,7 @@ class ASH_EXPORT WebNotificationTray
   bool IsMouseInNotificationBubble() const;
 
   message_center::MessageCenter* message_center() {
-    return message_center_.get();
+    return message_center_;
   }
 
   // Overridden from TrayBackgroundView.
@@ -152,7 +152,7 @@ class ASH_EXPORT WebNotificationTray
   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_;