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

services/ui/demo/mus_demo_internal.cc

 // 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.
 
 #include "services/ui/demo/mus_demo_internal.h"
 
 #include "services/service_manager/public/cpp/service_context.h"
 #include "services/ui/demo/window_tree_data.h"
 #include "ui/aura/mus/window_tree_client.h"
 #include "ui/aura/mus/window_tree_host_mus.h"
@@ skipping 30 matching lines @@
   return true;
 }
 
 bool MusDemoInternal::OnWmSetProperty(
     aura::Window* window,
     const std::string& name,
     std::unique_ptr<std::vector<uint8_t>>* new_data) {
   return true;
 }
 
+void MusDemoInternal::OnWmSetModalType(aura::Window* window, ModalType type) {}
+
 void MusDemoInternal::OnWmSetCanFocus(aura::Window* window, bool can_focus) {}
 
 aura::Window* MusDemoInternal::OnWmCreateTopLevelWindow(
     mojom::WindowType window_type,
     std::map<std::string, std::vector<uint8_t>>* properties) {
   NOTREACHED();
   return nullptr;
 }
 
 void MusDemoInternal::OnWmClientJankinessChanged(
@@ skipping 41 matching lines @@
     const std::vector<gfx::Rect>& additional_client_areas) {}
 
 bool MusDemoInternal::IsWindowActive(aura::Window* window) {
   return false;
 }
 
 void MusDemoInternal::OnWmDeactivateWindow(aura::Window* window) {}
 
 }  // namespace demo
 }  // namespace ui