Enable use_once_callback on mojom GN rule by default

content/test/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("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
@@ skipping 445 matching lines @@
   }
 
   configs += [ "//v8:external_startup_data" ]
 }
 
 mojom("content_test_mojo_bindings") {
   sources = [
     "../public/test/test_service.mojom",
     "test_browser_associated_interfaces.mojom",
   ]
-
-  use_once_callback = true
 }
 
 mojom("web_ui_test_mojo_bindings") {
   sources = [
     "data/web_ui_test_mojo_bindings.mojom",
   ]
-
-  use_once_callback = true
 }
 
 static_library("layouttest_support") {
   testonly = true
 
   # See comment at the top of //content/BUILD.gn for why this is disabled in
   # component builds.
   if (is_component_build) {
     check_includes = false
   }
@@ skipping 1351 matching lines @@
   if (is_android) {
     deps += [ "//testing/android/native_test:native_test_native_code" ]
   }
 }
 
 group("fuzzers") {
   deps = [
     "//content/test/fuzzer",
   ]
 }