Add service name constant for UI service.

services/ui/public/cpp/window_tree_host_factory.cc

 // Copyright 2015 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 "services/ui/public/cpp/window_tree_host_factory.h"
 
 #include "base/memory/ptr_util.h"
 #include "services/service_manager/public/cpp/connector.h"
 #include "services/ui/public/cpp/window_tree_client.h"
 #include "services/ui/public/cpp/window_tree_client_delegate.h"
+#include "services/ui/public/interfaces/constants.mojom.h"
 
 namespace ui {
 
 std::unique_ptr<WindowTreeClient> CreateWindowTreeHost(
     mojom::WindowTreeHostFactory* factory,
     WindowTreeClientDelegate* delegate,
     mojom::WindowTreeHostPtr* host,
     WindowManagerDelegate* window_manager_delegate) {
   mojom::WindowTreeClientPtr tree_client;
   std::unique_ptr<WindowTreeClient> window_tree_client =
       base::MakeUnique<WindowTreeClient>(delegate, window_manager_delegate,
                                          GetProxy(&tree_client));
   factory->CreateWindowTreeHost(GetProxy(host), std::move(tree_client));
   return window_tree_client;
 }
 
 std::unique_ptr<WindowTreeClient> CreateWindowTreeHost(
     service_manager::Connector* connector,
     WindowTreeClientDelegate* delegate,
     mojom::WindowTreeHostPtr* host,
     WindowManagerDelegate* window_manager_delegate) {
   mojom::WindowTreeHostFactoryPtr factory;
-  connector->ConnectToInterface("ui", &factory);
+  connector->ConnectToInterface(ui::mojom::kServiceName, &factory);
   return CreateWindowTreeHost(factory.get(), delegate, host,
                               window_manager_delegate);
 }
 
 }  // namespace ui