Add service name constant for UI service.

ash/mus/app_launch_unittest.cc

Index: ash/mus/app_launch_unittest.cc
diff --git a/ash/mus/app_launch_unittest.cc b/ash/mus/app_launch_unittest.cc
index c5999580fbd287be01a60e21699a7c73d501aa76..631eaf0b9d4355ba7969dece691c2b5db264b1f0 100644
--- a/ash/mus/app_launch_unittest.cc
+++ b/ash/mus/app_launch_unittest.cc
@@ -6,6 +6,7 @@
 #include "base/command_line.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"
 
 namespace ash {
@@ -44,7 +45,7 @@ TEST_F(AppLaunchTest, MAYBE_TestQuickLaunch) {
   connector()->Connect("quick_launch");
 
   ui::mojom::WindowServerTestPtr test_interface;
-  connector()->ConnectToInterface("ui", &test_interface);
+  connector()->ConnectToInterface(ui::mojom::kServiceName, &test_interface);
 
   base::RunLoop run_loop;
   bool success = false;