Add service name constant for UI service.

services/ui/demo/mus_demo_unittests.cc

Index: services/ui/demo/mus_demo_unittests.cc
diff --git a/services/ui/demo/mus_demo_unittests.cc b/services/ui/demo/mus_demo_unittests.cc
index 966e991c472b3986c79faf8cbe216cfa45595c2d..6f05aeda60812bda9ad96582e7fa9c9c9d2a2088 100644
--- a/services/ui/demo/mus_demo_unittests.cc
+++ b/services/ui/demo/mus_demo_unittests.cc
@@ -7,6 +7,7 @@
 #include "base/macros.h"
 #include "base/run_loop.h"
 #include "services/service_manager/public/cpp/service_test.h"
+#include "services/ui/public/interfaces/constants.mojom.h"
 #include "services/ui/public/interfaces/window_server_test.mojom.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ -42,7 +43,7 @@ TEST_F(MusDemoTest, CheckMusDemoDraws) {
   connector()->Connect("mus_demo");
 
   ::ui::mojom::WindowServerTestPtr test_interface;
-  connector()->ConnectToInterface("ui", &test_interface);
+  connector()->ConnectToInterface(ui::mojom::kServiceName, &test_interface);
 
   base::RunLoop run_loop;
   bool success = false;