Add a variable use_system_xcode to GN.

chrome/tools/build/mac/run_verify_order.py

 # Copyright 2016 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 argparse
 import os.path
 import sys
 import subprocess
 
 # Wraps chrome/tools/build/mac/verify_order for the GN build so that it can
 # write a stamp file, rather than operate as a postbuild.
 
 if __name__ == '__main__':
   parser = argparse.ArgumentParser(
       description='A wrapper around verify_order that writes a stamp file.')
   parser.add_argument('--stamp', action='store', type=str,
       help='Touch this stamp file on success.')
+  parser.add_argument('--developer_dir', required=False,
+                      help='Path to Xcode.')
 
   args, unknown_args = parser.parse_known_args()
+  if (args.developer_dir):
+    os.environ['DEVELOPER_DIR'] = args.developer_dir
 
   this_script_dir = os.path.dirname(sys.argv[0])
   rv = subprocess.check_call(
       [ os.path.join(this_script_dir, 'verify_order') ] + unknown_args)
 
   if rv == 0 and args.stamp:
     if os.path.exists(args.stamp):
       os.unlink(args.stamp)
     open(args.stamp, 'w+').close()
 
   sys.exit(rv)