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

ui/aura/mus/window_tree_client.cc

 // Copyright 2014 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 "ui/aura/mus/window_tree_client.h"
 
 #include <stddef.h>
 
 #include <string>
 #include <utility>
@@ skipping 1381 matching lines @@
           window->GetWindow(), name, data.get());
     }
   }
   if (window_manager_internal_client_)
     window_manager_internal_client_->WmResponse(change_id, result);
 }
 
 void WindowTreeClient::WmSetModalType(Id window_id, ui::ModalType type) {
   WindowMus* window = GetWindowByServerId(window_id);
   if (window)
-    window->GetWindow()->SetProperty(aura::client::kModalKey, type);
+    window_manager_delegate_->OnWmSetModalType(window->GetWindow(), type);
 }
 
 void WindowTreeClient::WmSetCanFocus(Id window_id, bool can_focus) {
   WindowMus* window = GetWindowByServerId(window_id);
   if (window)
     window_manager_delegate_->OnWmSetCanFocus(window->GetWindow(), can_focus);
 }
 
 void WindowTreeClient::WmCreateTopLevelWindow(
     uint32_t change_id,
@@ skipping 416 matching lines @@
   return ScheduleInFlightChange(base::MakeUnique<InFlightCaptureChange>(
       this, capture_synchronizer_.get(), window));
 }
 
 uint32_t WindowTreeClient::CreateChangeIdForFocus(WindowMus* window) {
   return ScheduleInFlightChange(base::MakeUnique<InFlightFocusChange>(
       this, focus_synchronizer_.get(), window));
 }
 
 }  // namespace aura