Add service name constant for UI service.

ui/aura/mus/window_tree_client.cc

Index: ui/aura/mus/window_tree_client.cc
diff --git a/ui/aura/mus/window_tree_client.cc b/ui/aura/mus/window_tree_client.cc
index ae5f69caf63bceb13573dc33ceb25a79942b1edb..a84e1e2f3c97cbfd89d7b88abfb30d4c2cb50f1f 100644
--- a/ui/aura/mus/window_tree_client.cc
+++ b/ui/aura/mus/window_tree_client.cc
@@ -14,6 +14,7 @@
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
 #include "services/service_manager/public/cpp/connector.h"
+#include "services/ui/public/interfaces/constants.mojom.h"
 #include "services/ui/public/interfaces/window_manager_window_tree_factory.mojom.h"
 #include "ui/aura/client/aura_constants.h"
 #include "ui/aura/client/drag_drop_client.h"
@@ -180,7 +181,7 @@ void WindowTreeClient::ConnectViaWindowTreeFactory(
   client_id_ = 101;
 
   ui::mojom::WindowTreeFactoryPtr factory;
-  connector->ConnectToInterface("ui", &factory);
+  connector->ConnectToInterface(ui::mojom::kServiceName, &factory);
   ui::mojom::WindowTreePtr window_tree;
   factory->CreateWindowTree(GetProxy(&window_tree),
                             binding_.CreateInterfacePtrAndBind());
@@ -192,7 +193,7 @@ void WindowTreeClient::ConnectAsWindowManager(
   DCHECK(window_manager_delegate_);
 
   ui::mojom::WindowManagerWindowTreeFactoryPtr factory;
-  connector->ConnectToInterface("ui", &factory);
+  connector->ConnectToInterface(ui::mojom::kServiceName, &factory);
   ui::mojom::WindowTreePtr window_tree;
   factory->CreateWindowTree(GetProxy(&window_tree),
                             binding_.CreateInterfacePtrAndBind());