[infra] Prepare gn.py for update to GN binary

tools/gn.py

 #!/usr/bin/env python
 # Copyright 2016 The Dart project 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 argparse
 import multiprocessing
 import os
 import subprocess
 import sys
@@ skipping 349 matching lines @@
       default=use_wheezy(),
       action='store_true')
   other_group.add_argument('--no-wheezy',
       help='Disable the Debian wheezy sysroot on Linux',
       dest='wheezy',
       action='store_false')
   other_group.add_argument('--workers', '-w',
       type=int,
       help='Number of simultaneous GN invocations',
       dest='workers',
-      default=multiprocessing.cpu_count())
+      # Set to multiprocessing.cpu_count() when GN can be run in parallel.
+      default=1)
 
   options = parser.parse_args(args)
   if not process_options(options):
     parser.print_help()
     return None
   return options
 
 
 def run_command(command):
   try:
@@ skipping 46 matching lines @@
       return 1
 
   endtime = time.time()
   if args.verbose:
     print ("GN Time: %.3f seconds" % (endtime - starttime))
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))