Enable use_once_callback on mojom GN rule by default

ui/events/mojo/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")
 
 mojom("interfaces") {
   sources = [
     "event.mojom",
     "event_constants.mojom",
     "keyboard_codes.mojom",
   ]
 
   public_deps = [
     "//ui/gfx/mojo",
     "//ui/latency/mojo:interfaces",
   ]
 }
 
 mojom("test_interfaces") {
   testonly = true
   sources = [
     "traits_test_service.mojom",
   ]
 
   public_deps = [
     ":interfaces",
   ]
+
+  # TODO(crbug.com/714018): Convert the implementation to use OnceCallback.
+  use_once_callback = false
 }