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

mash/simple_wm/simple_wm.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 MASH_SIMPLE_WM_SIMPLE_WM_H_
 #define MASH_SIMPLE_WM_SIMPLE_WM_H_
 
 #include <memory>
 #include <utility>
 
@@ skipping 58 matching lines @@
                               aura::Window* target) override;
   aura::PropertyConverter* GetPropertyConverter() override;
 
   // aura::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 janky) 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 34 matching lines @@
   std::unique_ptr<WorkspaceLayoutManager> workspace_layout_manager_;
 
   bool started_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(SimpleWM);
 };
 
 }  // namespace simple_wm
 
 #endif  // MASH_SIMPLE_WM_SIMPLE_WM_H_