Reland Enable hermetic builds for iOS.

build/mac/should_use_hermetic_xcode.py

 #!/usr/bin/env 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.
 
 """Prints "1" if Chrome targets should be built with hermetic xcode. Otherwise
-prints "0"."""
+prints "0".
+
+Usage:
+  python should_use_hermetic_xcode.py <target_os>
+"""
 
 import os
 import sys
 
 
 def _IsCorpMachine():
   return os.path.isdir('/Library/GoogleCorpSupport/')
 
 
 def main():
-  if os.environ.get('FORCE_MAC_TOOLCHAIN') or _IsCorpMachine():
+  allow_corp = sys.argv[1] == 'mac' and _IsCorpMachine()
+  if os.environ.get('FORCE_MAC_TOOLCHAIN') or allow_corp:
     return "1"
   else:
     return "0"
 
 
 if __name__ == '__main__':
   print main()
   sys.exit(0)