[Athena] minimum impl to add screen/background and test windows

athena/wm/public/window_manager.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_SCREEN_PUBLIC_WINDOW_MANAGER_H_
+#define ATHENA_SCREEN_PUBLIC_WINDOW_MANAGER_H_
+
+#include "athena/athena_export.h"
+
+namespace athena {
+
+class ATHENA_EXPORT WindowManager {
+ public:
+  static WindowManager* Create();
+  static void Shutdown();
+
+  virtual ~WindowManager() {}
+};
+
+}  // namespace athena
+
+#endif  // ATHENA_SCREEN_PUBLIC_WINDOW_MANAGER_H_