aura-mus: Implement stacking in DesktopWindowTreeHostMus.

ui/views/mus/mus_client_test_observer.h

+// Copyright 2017 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 UI_VIEWS_MUS_MUS_CLIENT_TEST_OBSERVER_H_
+#define UI_VIEWS_MUS_MUS_CLIENT_TEST_OBSERVER_H_
+
+#include "ui/views/mus/mus_export.h"
+
+namespace views {
+namespace test {
+
+// Testing interface for MusClient internals.
+class VIEWS_MUS_EXPORT MusClientTestObserver {
+ public:
+  virtual void OnChangeCompleted(aura::ChangeType type, bool success) = 0;
+
+ protected:
+  virtual ~MusClientTestObserver() {}
+};
+
+}  // namespace test
+}  // namespace views
+
+#endif  // UI_VIEWS_MUS_MUS_CLIENT_TEST_OBSERVER_H_