Reland: mus: Enable system modal windows

components/mus/ws/server_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/ws/server_window.h"
 
 #include <inttypes.h>
 #include <stddef.h>
 
 #include "base/strings/stringprintf.h"
@@ skipping 211 matching lines @@
   for (ServerWindow* child : children_) {
     ServerWindow* window = child->GetChildWindow(window_id);
     if (window)
       return window;
   }
 
   return nullptr;
 }
 
 void ServerWindow::AddTransientWindow(ServerWindow* child) {
+  // A system modal window cannot become a transient child.
+  DCHECK(!child->is_modal() || child->transient_parent());
+
   if (child->transient_parent())
     child->transient_parent()->RemoveTransientWindow(child);
 
   DCHECK(std::find(transient_children_.begin(), transient_children_.end(),
                    child) == transient_children_.end());
   transient_children_.push_back(child);
   child->transient_parent_ = this;
 
   // Restack |child| properly above its transient parent, if they share the same
   // parent.
@@ skipping 217 matching lines @@
 }
 
 // static
 ServerWindow** ServerWindow::GetStackingTarget(ServerWindow* window) {
   return &window->stacking_target_;
 }
 
 }  // namespace ws
 
 }  // namespace mus