Adding functions to the window_list_provider for accessing the activities window list

athena/wm/public/window_list_provider_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_WM_PUBLIC_WINDOW_LIST_PROVIDER_OBSERVER_H_
+#define ATHENA_WM_PUBLIC_WINDOW_LIST_PROVIDER_OBSERVER_H_
+
+#include "athena/athena_export.h"
+
+namespace athena {
+
+// An observer to window list changes like e.g. stacking order has changed.
+class ATHENA_EXPORT WindowListProviderObserver {
+ public:
+  virtual ~WindowListProviderObserver() {}
+
+  // The Window stacking has changed.
+  virtual void OnWindowStackingChanged() = 0;
+};
+
+}  // namespace athena
+
+#endif  // ATHENA_WM_PUBLIC_WINDOW_LIST_PROVIDER_OBSERVER_H_