Add service name constant for UI service.

services/ui/ws/window_tree_client_unittest.cc

Index: services/ui/ws/window_tree_client_unittest.cc
diff --git a/services/ui/ws/window_tree_client_unittest.cc b/services/ui/ws/window_tree_client_unittest.cc
index eefa04268c4ce417599f5f871cb5104aa935f863..4b9232f9fd64d325b352575315603575d94929c5 100644
--- a/services/ui/ws/window_tree_client_unittest.cc
+++ b/services/ui/ws/window_tree_client_unittest.cc
@@ -16,6 +16,7 @@
 #include "services/service_manager/public/cpp/interface_registry.h"
 #include "services/service_manager/public/cpp/service_test.h"
 #include "services/ui/public/cpp/tests/window_server_shelltest_base.h"
+#include "services/ui/public/interfaces/constants.mojom.h"
 #include "services/ui/public/interfaces/window_tree.mojom.h"
 #include "services/ui/public/interfaces/window_tree_host.mojom.h"
 #include "services/ui/ws/ids.h"
@@ -650,7 +651,7 @@ class WindowTreeClientTest : public WindowServerServiceTestBase {
     WindowServerServiceTestBase::SetUp();
 
     mojom::WindowTreeHostFactoryPtr factory;
-    connector()->ConnectToInterface("ui", &factory);
+    connector()->ConnectToInterface(ui::mojom::kServiceName, &factory);
 
     mojom::WindowTreeClientPtr tree_client_ptr;
     wt_client1_ = base::MakeUnique<TestWindowTreeClient>();