Refators display related functionality into own class

components/mus/ws/window_manager_state.cc

 // Copyright 2016 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/window_manager_state.h"
 
 #include "components/mus/ws/connection_manager.h"
+#include "components/mus/ws/display_manager.h"
 #include "components/mus/ws/server_window.h"
 #include "mojo/shell/public/interfaces/connector.mojom.h"
 
 namespace mus {
 namespace ws {
 
 WindowManagerState::WindowManagerState(Display* display)
     : WindowManagerState(display, false, mojo::shell::mojom::kRootUserID) {}
 
 WindowManagerState::WindowManagerState(Display* display, const UserId& user_id)
     : WindowManagerState(display, true, user_id) {}
 
 WindowManagerState::~WindowManagerState() {}
 
 WindowManagerState::WindowManagerState(Display* display,
                                        bool is_user_id_valid,
                                        const UserId& user_id)
     : display_(display),
       is_user_id_valid_(is_user_id_valid),
       user_id_(user_id) {
   ConnectionManager* connection_manager = display_->connection_manager();
   root_.reset(connection_manager->CreateServerWindow(
-      RootWindowId(connection_manager->GetAndAdvanceNextRootId()),
+      connection_manager->display_manager()->GetAndAdvanceNextRootId(),
       ServerWindow::Properties()));
   // Our root is always a child of the Display's root. Do this
   // before the WindowTree has been created so that the client doesn't get
   // notified of the add, bounds change and visibility change.
   root_->SetBounds(gfx::Rect(display->root_window()->bounds().size()));
   root_->SetVisible(true);
   display->root_window()->Add(root_.get());
 }
 
 }  // namespace ws
 }  // namespace mus