Reland: mus: Enable system modal windows

components/mus/public/cpp/lib/window.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 "components/mus/public/cpp/window.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <set>
@@ skipping 325 matching lines @@
   if (connection_)
     CHECK_EQ(child->connection_, connection_);
   for (const Window* p = child->parent(); p; p = p->parent()) {
     if (p == this)
       return true;
   }
   return false;
 }
 
 void Window::AddTransientWindow(Window* transient_window) {
+  // A system modal window cannot become a transient child.
+  DCHECK(!transient_window->is_modal() || transient_window->transient_parent());
+
   if (connection_)
     CHECK_EQ(transient_window->connection(), connection_);
   LocalAddTransientWindow(transient_window);
   if (connection_)
     tree_client()->AddTransientWindow(this, transient_window->server_id());
 }
 
 void Window::RemoveTransientWindow(Window* transient_window) {
   if (connection_)
     CHECK_EQ(transient_window->connection(), connection_);
@@ skipping 511 matching lines @@
     notifier->NotifyWindowReordered();
 
   return true;
 }
 
 // static
 Window** Window::GetStackingTarget(Window* window) {
   return &window->stacking_target_;
 }
 }  // namespace mus