athena: Expose the list of activities from the ActivityManager.

athena/activity/public/activity_manager_observer.h

 // Copyright 2014 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 ATHENA_ACTIVITY_PUBLIC_ACTIVITY_MANAGER_OBSERVER_H_
 #define ATHENA_ACTIVITY_PUBLIC_ACTIVITY_MANAGER_OBSERVER_H_
 
 #include "athena/athena_export.h"
 
 namespace athena {
 
 class Activity;
 
 class ATHENA_EXPORT ActivityManagerObserver {
  public:
   virtual ~ActivityManagerObserver() {}
 
   // Called after an |activity| got created.
   virtual void OnActivityStarted(Activity* activity) = 0;
 
   // Called before an |activity| gets destroyed.
   virtual void OnActivityEnding(Activity* activity) = 0;
+
+  // Called when the order of the activities in the list has changed.
+  virtual void OnActivityOrderChanged() = 0;
 };
 
 }  // namespace athena
 
 #endif  // ATHENA_ACTIVITY_PUBLIC_ACTIVITY_MANAGER_OBSERVER_H_