Make MessageCenter a singleton object and build on Windows.

ui/message_center/message_center.h

Index: ui/message_center/message_center.h
diff --git a/ui/message_center/message_center.h b/ui/message_center/message_center.h
index bde776743ed614bce36afb0a8bf41d9a68fa050a..672123fd08e91e0e861b37b7eacb9368086e2d4b 100644
--- a/ui/message_center/message_center.h
+++ b/ui/message_center/message_center.h
@@ -10,6 +10,8 @@
 #include "ui/message_center/notification_list.h"
 #include "ui/notifications/notification_types.h"
 
+template <typename T> struct DefaultSingletonTraits;
+
 namespace base {
 class DictionaryValue;
 }
@@ -70,11 +72,13 @@ class MESSAGE_CENTER_EXPORT MessageCenter : public NotificationList::Delegate {
     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);
 
@@ -133,6 +137,9 @@ class MESSAGE_CENTER_EXPORT MessageCenter : public NotificationList::Delegate {
   virtual NotificationList* GetNotificationList() OVERRIDE;
 
  private:
+  MessageCenter();
+  friend struct DefaultSingletonTraits<MessageCenter>;
+
   scoped_ptr<NotificationList> notification_list_;
   Host* host_;
   Delegate* delegate_;