Add a variable use_system_xcode to GN.

build/config/mac/xcrun.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
 import subprocess
 import sys
 
 if __name__ == '__main__':
   parser = argparse.ArgumentParser(
       description='A script to execute a command via xcrun.')
   parser.add_argument('--stamp', action='store', type=str,
       help='Write a stamp file to this path 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
+
   rv = subprocess.check_call(['xcrun'] + 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)