aura-mus: Implement stacking in DesktopWindowTreeHostMus.

ui/aura/mus/window_tree_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_AURA_MUS_WINDOW_TREE_CLIENT_TEST_OBSERVER_H_
+#define UI_AURA_MUS_WINDOW_TREE_CLIENT_TEST_OBSERVER_H_
+
+#include <cstdint>
+
+#include "ui/aura/aura_export.h"
+
+namespace aura {
+enum class ChangeType;
+
+class AURA_EXPORT WindowTreeClientTestObserver {
+ public:
+  // Called when a new change id is created.
+  virtual void OnChangeStarted(uint32_t change_id, ChangeType type) = 0;
+
+  // Called when a change is completed, successfully or unsuccessfully.
+  virtual void OnChangeCompleted(uint32_t change_id,
+                                 ChangeType type, bool success) = 0;
+
+ protected:
+  virtual ~WindowTreeClientTestObserver() {}
+};
+
+}  // namespace aura
+
+#endif  // UI_AURA_MUS_WINDOW_TREE_CLIENT_TEST_OBSERVER_H_