Make MessageCenter a singleton object and build on Windows.

ui/message_center/message_center.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 UI_MESSAGE_CENTER_MESSAGE_CENTER_H_
 #define UI_MESSAGE_CENTER_MESSAGE_CENTER_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "ui/message_center/message_center_export.h"
 #include "ui/message_center/notification_list.h"
 #include "ui/notifications/notification_types.h"
 
+template <typename T> struct DefaultSingletonTraits;
+
 namespace base {
 class DictionaryValue;
 }
 
 // Class for managing the NotificationList. The client (e.g. Chrome) calls
 // [Add|Remove|Update]Notification to create and update notifications in the
 // list. It can also implement Delegate to receive callbacks when a
 // notification is removed (closed), or clicked on.
 // If a Host is provided, it will be informed when the notification list
 // changes, and is expected to handle creating, showing, and hiding of any
@@ skipping 40 matching lines @@
     //
     // TODO(miket): consider providing default implementations for the pure
     // virtuals above, to avoid changing so many files in disparate parts of
     // the codebase each time we enhance this interface.
     virtual void OnButtonClicked(const std::string& id, int button_index);
 
    protected:
     virtual ~Delegate() {}
   };
 
-  // |host| is expected to manage any notification bubbles. It may be NULL.
-  explicit MessageCenter(Host* host);
+  static MessageCenter* GetInstance();
 
   virtual ~MessageCenter();
 
+  // Called once to set the host, which is expected to manage any notification
+  // bubbles.  |host| can be NULL.

[stevenjb, 2012/12/27 19:49:34]

I would remove the '|host| can be NULL' part of the comment. The code won't
crash, but that doesn't seem like meaningful usage.

[dewittj, 2012/12/27 23:38:50]

Done.

+  void SetHost(Host* host);
   // Called once to set the delegate.
   void SetDelegate(Delegate* delegate);
 
   // Informs the notification list whether the message center is visible.
   // This affects whether or not a message has been "read".
   void SetMessageCenterVisible(bool visible);
 
   // Accessors to notification_list_
   size_t NotificationCount() const;
   size_t UnreadNotificationCount() const;
@@ skipping 38 matching lines @@
   virtual void DisableNotificationByExtension(const std::string& id) OVERRIDE;
   virtual void DisableNotificationByUrl(const std::string& id) OVERRIDE;
   virtual void ShowNotificationSettings(const std::string& id) OVERRIDE;
   virtual void OnNotificationClicked(const std::string& id) OVERRIDE;
   virtual void OnQuietModeChanged(bool quiet_mode) OVERRIDE;
   virtual void OnButtonClicked(const std::string& id, int button_index)
       OVERRIDE;
   virtual NotificationList* GetNotificationList() OVERRIDE;
 
  private:
+  MessageCenter();
+  friend struct DefaultSingletonTraits<MessageCenter>;
+
   scoped_ptr<NotificationList> notification_list_;
   Host* host_;
   Delegate* delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageCenter);
 };
 
 }  // namespace message_center
 
 #endif  // UI_MESSAGE_CENTER_MESSAGE_CENTER_H_