mus: Send display-id to client only for the root 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 590 matching lines @@
 void Window::LocalDestroy() {
   delete this;
 }
 
 void Window::LocalAddChild(Window* child) {
   ScopedTreeNotifier notifier(child, child->parent(), this);
   if (child->parent())
     RemoveChildImpl(child, &child->parent_->children_);
   children_.push_back(child);
   child->parent_ = this;
+  child->display_id_ = display_id_;
 }
 
 void Window::LocalRemoveChild(Window* child) {
   DCHECK_EQ(this, child->parent());
   ScopedTreeNotifier notifier(child, this, nullptr);
   RemoveChildImpl(child, &children_);
 }
 
 void Window::LocalAddTransientWindow(Window* transient_window) {
   if (transient_window->transient_parent())
@@ skipping 264 matching lines @@
     notifier->NotifyWindowReordered();
 
   return true;
 }
 
 // static
 Window** Window::GetStackingTarget(Window* window) {
   return &window->stacking_target_;
 }
 }  // namespace mus