Add service name constant for UI service.

services/ui/public/cpp/window_tree_host_factory.cc

Index: services/ui/public/cpp/window_tree_host_factory.cc
diff --git a/services/ui/public/cpp/window_tree_host_factory.cc b/services/ui/public/cpp/window_tree_host_factory.cc
index e06bd3c78e51dc4a18d4c4549f8309ed5f20e79d..c78dbe1f21895e7415949aa4495ac16c8762b8bf 100644
--- a/services/ui/public/cpp/window_tree_host_factory.cc
+++ b/services/ui/public/cpp/window_tree_host_factory.cc
@@ -8,6 +8,7 @@
 #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 {
 
@@ -30,7 +31,7 @@ std::unique_ptr<WindowTreeClient> CreateWindowTreeHost(
     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);
 }