Enable use_once_callback on mojom GN rule by default

headless/BUILD.gn

 # Copyright 2015 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("//build/config/chrome_build.gni")
 import("//build/util/process_version.gni")
 import("//headless/headless.gni")
 import("//mojo/public/tools/bindings/mojom.gni")
 import("//printing/features/features.gni")
 import("//testing/test.gni")
@@ skipping 88 matching lines @@
 
   deps = [
     ":pak",
   ]
 }
 
 mojom("tab_socket") {
   sources = [
     "lib/tab_socket.mojom",
   ]
+
+  # TODO(crbug.com/714018): Convert the implementation to use OnceCallback.
+  use_once_callback = false
 }
 
 grit("resources") {
   source = "lib/resources/headless_lib_resources.grd"
   outputs = [
     "grit/headless_lib_resources.h",
     "$root_gen_dir/headless/headless_lib_resources.pak",
   ]
   grit_flags = [
     "-E",
@@ skipping 402 matching lines @@
 
 executable("headless_example") {
   sources = [
     "app/headless_example.cc",
   ]
 
   deps = [
     ":headless_shell_lib",
   ]
 }