Add service name constant for UI service.

services/ui/public/interfaces/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//testing/test.gni")
 
 mojom("interfaces") {
   sources = [
     "accessibility_manager.mojom",
@@ skipping 16 matching lines @@
     "window_tree_constants.mojom",
     "window_tree_host.mojom",
   ]
 
   import_dirs = [
     get_path_info("../../../..", "abspath"),
     "//mojo/services",
   ]
 
   public_deps = [
+    ":constants",
     "//cc/ipc:interfaces",
     "//gpu/ipc/common:interfaces",
     "//services/ui/public/interfaces/display",
     "//ui/events/mojo:interfaces",
     "//ui/gfx/geometry/mojo",
     "//ui/gfx/mojo",
     "//ui/platform_window/mojo:interfaces",
   ]
 
   use_new_wrapper_types = false
 }
 
+mojom("constants") {
+  sources = [
+    "constants.mojom",
+  ]
+}
+
 test("ui_struct_traits_unittests") {
   testonly = true
 
   sources = [
     "display/display_struct_traits_unittest.cc",
   ]
 
   deps = [
     "//base",
     "//base/test:test_support",
     "//mojo/edk/test:run_all_unittests",
     "//services/ui/public/interfaces/display:test_interfaces",
     "//testing/gtest",
     "//ui/display/types",
     "//ui/gfx:test_support",
   ]
 }