Add option to allow mojob to specify extra gn args.

mojo/public/tools/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/module_args/mojo.gni")
 import("../mojo.gni")
 
 if (use_prebuilt_mojo_shell) {
   copy("copy_mojo_shell") {
     filename = "mojo_shell"
@@ skipping 13 matching lines @@
       outputs = [
         "$root_out_dir/$filename",
       ]
     }
   }
 }
 
 if (use_prebuilt_network_service) {
   copy("copy_network_service") {
     filename = "network_service.mojo"
-    if (is_android) {
+    if (defined(prebuilt_network_service_location) &&
+        prebuilt_network_service_location != "") {
       sources = [
-        "prebuilt/network_service/android-arm/$filename",
+        "$prebuilt_network_service_location",
       ]
     } else {
-      assert(is_linux)
-      sources = [
-        "prebuilt/network_service/linux-x64/$filename",
-      ]
+      if (is_android) {
+        assert(cpu_arch == "arm",
+               "Only arm version prebuilt netowrk_service.mojo is available.")
+        sources = [
+          "prebuilt/network_service/android-arm/$filename",
+        ]
+      } else {
+        assert(is_linux)
+        sources = [
+          "prebuilt/network_service/linux-x64/$filename",
+        ]
+      }
     }
+
     outputs = [
       "$root_out_dir/$filename",
     ]
   }
 
   copy("copy_network_service_apptests") {
     filename = "network_service_apptests.mojo"
     if (is_android) {
       sources = [
         "prebuilt/network_service_apptests/android-arm/$filename",
@@ skipping 15 matching lines @@
 # "dart_package" template in mojo/public/dart/rules.gni can introspect on,
 # accessing the 'label' and 'target_out_dir' variables.
 copy("dart_apptest_framework") {
   sources = [
     "prebuilt/frameworks/apptest.dartzip",
   ]
   outputs = [
     "$target_out_dir/dart_apptest_framework.dartzip",
   ]
 }