Cleanup remaining class/struct fwd declarations

apps/app_lifetime_monitor.h

 // Copyright 2013 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 APPS_APP_LIFETIME_MONITOR_H_
 #define APPS_APP_LIFETIME_MONITOR_H_
 
 #include <string>
 #include <vector>
 
 #include "base/observer_list.h"
 #include "components/keyed_service/core/keyed_service.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 #include "extensions/browser/app_window/app_window_registry.h"
 
-namespace extensions {
-class Extension;
-}
-
 class Profile;
 
 namespace apps {
 
 // Observes startup of apps and their windows and notifies observers of these
 // events.
 class AppLifetimeMonitor : public KeyedService,
                            public content::NotificationObserver,
                            public extensions::AppWindowRegistry::Observer {
  public:
@@ skipping 51 matching lines @@
   content::NotificationRegistrar registrar_;
   Profile* profile_;
   base::ObserverList<Observer> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(AppLifetimeMonitor);
 };
 
 }  // namespace apps
 
 #endif  // APPS_APP_LIFETIME_MONITOR_H_