aura-mus: Implement stacking in DesktopWindowTreeHostMus.

ui/views/mus/test_utils.h

 // Copyright 2016 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_TEST_UTILS_H_
 #define UI_VIEWS_MUS_TEST_UTILS_H_
 
 #include "base/memory/ptr_util.h"
 #include "ui/views/mus/mus_client.h"
 
 namespace views {
 namespace test {
 
 class MusClientTestApi {
  public:
   static std::unique_ptr<MusClient> Create(
       service_manager::Connector* connector,
       const service_manager::Identity& identity) {
     return base::WrapUnique(new MusClient(connector, identity));
   }
 
+  static void SetChangeTestObserver(MusClientTestObserver* observer) {
+    MusClient::Get()->test_observer_ = observer;
+  }
+
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(MusClientTestApi);
 };
 
 }  // namespace test
 }  // namespace views
 
 #endif  // UI_VIEWS_MUS_TEST_UTILS_H_