Add system modals to the proper container in mus+ash.

ui/aura/test/aura_test_base.h

 // Copyright (c) 2012 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_TEST_AURA_TEST_BASE_H_
 #define UI_AURA_TEST_AURA_TEST_BASE_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 84 matching lines @@
   void OnPointerEventObserved(const ui::PointerEvent& event,
                               Window* target) override;
 
   // WindowManagerDelegate:
   void SetWindowManagerClient(WindowManagerClient* client) override;
   bool OnWmSetBounds(Window* window, gfx::Rect* bounds) override;
   bool OnWmSetProperty(
       Window* window,
       const std::string& name,
       std::unique_ptr<std::vector<uint8_t>>* new_data) override;
+  void OnWmSetModalType(Window* window, ui::ModalType type) override;
   void OnWmSetCanFocus(Window* window, bool can_focus) override;
   Window* OnWmCreateTopLevelWindow(
       ui::mojom::WindowType window_type,
       std::map<std::string, std::vector<uint8_t>>* properties) override;
   void OnWmClientJankinessChanged(const std::set<Window*>& client_windows,
                                   bool janky) override;
   void OnWmWillCreateDisplay(const display::Display& display) override;
   void OnWmNewDisplay(std::unique_ptr<WindowTreeHostMus> window_tree_host,
                       const display::Display& display) override;
   void OnWmDisplayRemoved(WindowTreeHostMus* window_tree_host) override;
@@ skipping 54 matching lines @@
   void SetUp() override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(AuraTestBaseMus);
 };
 
 }  // namespace test
 }  // namespace aura
 
 #endif  // UI_AURA_TEST_AURA_TEST_BASE_H_