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

ash/mus/window_manager.h

 // Copyright 2015 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 ASH_MUS_WINDOW_MANAGER_H_
 #define ASH_MUS_WINDOW_MANAGER_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 126 matching lines @@
                               aura::Window* target) override;
   aura::PropertyConverter* GetPropertyConverter() override;
 
   // WindowManagerDelegate:
   void SetWindowManagerClient(aura::WindowManagerClient* client) override;
   bool OnWmSetBounds(aura::Window* window, gfx::Rect* bounds) override;
   bool OnWmSetProperty(
       aura::Window* window,
       const std::string& name,
       std::unique_ptr<std::vector<uint8_t>>* new_data) override;
+  void OnWmSetModalType(aura::Window* window, ui::ModalType type) override;
   void OnWmSetCanFocus(aura::Window* window, bool can_focus) override;
   aura::Window* OnWmCreateTopLevelWindow(
       ui::mojom::WindowType window_type,
       std::map<std::string, std::vector<uint8_t>>* properties) override;
   void OnWmClientJankinessChanged(const std::set<aura::Window*>& client_windows,
                                   bool not_responding) override;
   void OnWmWillCreateDisplay(const display::Display& display) override;
   void OnWmNewDisplay(std::unique_ptr<aura::WindowTreeHostMus> window_tree_host,
                       const display::Display& display) override;
   void OnWmDisplayRemoved(aura::WindowTreeHostMus* window_tree_host) override;
@@ skipping 42 matching lines @@
   // See WmShellMus's constructor for details. Tests may set to false.
   bool create_session_state_delegate_stub_for_test_ = true;
 
   DISALLOW_COPY_AND_ASSIGN(WindowManager);
 };
 
 }  // namespace mus
 }  // namespace ash
 
 #endif  // ASH_MUS_WINDOW_MANAGER_H_