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 <string>
+
 #include "base/memory/scoped_ptr.h"
+#include "base/observer_list.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
+// If an Observer is provided, it will be informed when the notification list
 // changes, and is expected to handle creating, showing, and hiding of any
 // bubbles.
 
 namespace message_center {
 
 class MESSAGE_CENTER_EXPORT MessageCenter : public NotificationList::Delegate {
  public:
   // Class that hosts the message center.
-  class MESSAGE_CENTER_EXPORT Host {
+  class MESSAGE_CENTER_EXPORT Observer {
    public:
     // Called when the notification list has changed. |new_notification| will
     // be true if a notification was added or updated.
-    virtual void MessageCenterChanged(bool new_notification) = 0;
-
+    virtual void OnMessageCenterChanged(bool new_notification) = 0;
    protected:
-    virtual ~Host() {}
+    virtual ~Observer() {}
   };
 
   class MESSAGE_CENTER_EXPORT Delegate {
    public:
     // Called when the notification associated with |notification_id| is
     // removed (i.e. closed by the user).
     virtual void NotificationRemoved(const std::string& notification_id) = 0;
 
     // Request to disable the extension associated with |notification_id|.
     virtual void DisableExtension(const std::string& notification_id) = 0;
@@ skipping 15 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 delegate.
+  // Called to set the delegate.  Note that since the MessageCenter is a
+  // singleton object, it may already be created and have notifications in its
+  // NotificationList.

[stevenjb, 2013/01/03 22:32:45]

I would change this to something like:
// Called to set the delegate. Generally called only once, except in tests.
Changing the delegate does not affect notifications in its NotificationList.

[dewittj, 2013/01/04 00:18:14]

Done.

   void SetDelegate(Delegate* delegate);
 
+  // Management of the observer list.
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
   // 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;
   bool HasPopupNotifications() const;
 
   // Adds a new notification. |id| is a unique identifier, used to update or
@@ skipping 37 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:
+  friend struct DefaultSingletonTraits<MessageCenter>;
+  MessageCenter();
+
+  // Calls OnMessageCenterChanged on each observer.
+  void NotifyMessageCenterChanged(bool new_notification);
+
   scoped_ptr<NotificationList> notification_list_;
-  Host* host_;
+  ObserverList<Observer> observer_list_;
   Delegate* delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(MessageCenter);
 };
 
 }  // namespace message_center
 
 #endif  // UI_MESSAGE_CENTER_MESSAGE_CENTER_H_