Add service name constant for UI service.

services/ui/public/interfaces/BUILD.gn

Index: services/ui/public/interfaces/BUILD.gn
diff --git a/services/ui/public/interfaces/BUILD.gn b/services/ui/public/interfaces/BUILD.gn
index 0b2d7bb2ec9d98efcf671e11c22f6bb35fadbbfc..dfd9be4782457a47e451e272e25465b40c9da067 100644
--- a/services/ui/public/interfaces/BUILD.gn
+++ b/services/ui/public/interfaces/BUILD.gn
@@ -34,6 +34,7 @@ mojom("interfaces") {
   ]
 
   public_deps = [
+    ":constants",
     "//cc/ipc:interfaces",
     "//gpu/ipc/common:interfaces",
     "//services/ui/public/interfaces/display",
@@ -46,6 +47,12 @@ mojom("interfaces") {
   use_new_wrapper_types = false
 }
 
+mojom("constants") {
+  sources = [
+    "constants.mojom",
+  ]
+}
+
 test("ui_struct_traits_unittests") {
   testonly = true