Support of specify local prebuilt network service apptests file.

build/module_args/mojo.gni

 # 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.
 
 # This variable should point to the parent directory of the Mojo SDK.
 mojo_sdk_root = "//third_party/mojo/src"
 
 # Chromium builds the network service from source, as it is the
 # producer of the network service.
 mojo_build_network_service_from_source = true
@@ skipping 11 matching lines @@
   #
   # This variable only used when mojo_build_network_service_from_source is
   # false.
   #
   # Currently only android_arm and linux_64 available on gs://mojo, and no plan
   # to support more(https://codereview.chromium.org/921873003).
   # It is needed for developers works on other platforms like android_x86 and
   # android_x64. And it is also useful for supportted platform as well when
   # developer want to try its own version of network service files.
   mojo_prebuilt_network_service_location = ""
+
+  # For network_service_apptests.mojo
+  mojo_prebuilt_network_service_apptests_location = ""
 }