Correct iOS GN hermetic support.

ios/build/tools/setup-gn.py

 #!/usr/bin/python
 # 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 convert_gn_xcodeproj
 import errno
 import os
 import re
@@ skipping 78 matching lines @@
       goma_dir = self._settings.getstring('goma', 'install')
       if goma_dir:
         args.append(('goma_dir', '"%s"' % os.path.expanduser(goma_dir)))
 
     args.append(('is_debug', self._config == 'Debug'))
     args.append(('enable_dsyms', self._config in ('Profile', 'Official')))
     args.append(('enable_stripping', 'enable_dsyms'))
     args.append(('is_official_build', self._config == 'Official'))
     args.append(('is_chrome_branded', 'is_official_build'))
     args.append(('use_xcode_clang', 'is_official_build'))
+    if os.environ['FORCE_MAC_TOOLCHAIN']:
+      args.append(('use_system_xcode', False))
 
     cpu_values = self.TARGET_CPU_VALUES[self._target]
     build_arch = self._settings.getstring('build', 'arch')
     if build_arch == 'fat':
       target_cpu = cpu_values[self.FAT_BUILD_DEFAULT_ARCH]
       args.append(('target_cpu', target_cpu))
       args.append(('additional_target_cpus',
           [cpu for cpu in cpu_values.itervalues() if cpu != target_cpu]))
     else:
       args.append(('target_cpu', cpu_values[build_arch]))
@@ skipping 228 matching lines @@
   out_dir = os.path.join(args.root, 'out')
   if not os.path.isdir(out_dir):
     os.makedirs(out_dir)
 
   GenerateXcodeProject(gn_path, args.root, out_dir, settings)
   GenerateGnBuildRules(gn_path, args.root, out_dir, settings)
 
 
 if __name__ == '__main__':
   sys.exit(Main(sys.argv[1:]))