Fix buggy usage of global variables in the build system

mojo/public/mojo.gni

 # 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")
 
 # If using the prebuilt shell, gate its usage by the platforms for which it is
 # published.
-if (!defined(use_prebuilt_mojo_shell) || use_prebuilt_mojo_shell) {
+use_prebuilt_mojo_shell = false
+if (!defined(build_mojo_shell_from_source) || !build_mojo_shell_from_source) {
   use_prebuilt_mojo_shell = is_linux || is_android
 }
 
 # If using the prebuilt network service, gate its usage by the platforms for
 # which it is published.
-if (!defined(use_prebuilt_network_service) || use_prebuilt_network_service) {
+use_prebuilt_network_service = false
+if (!defined(build_network_service_from_source) ||
+    !build_network_service_from_source) {
   use_prebuilt_network_service = is_linux || is_android
 }
 
 # The absolute path to the directory containing the mojo public SDK (i.e., the
 # directory containing mojo/public). The build files within the Mojo public
 # SDK use this variable to allow themselves to be parameterized by the location
 # of the public SDK within a client repo.
 mojo_root = get_path_info("../..", "abspath")