Add canned presubmit check for GN formatting

gn.py

 #!/usr/bin/env python
 # Copyright 2013 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.
 
 """This script is a wrapper around the GN binary that is pulled from Google
 Cloud Storage when you sync Chrome. The binaries go into platform-specific
 subdirectories in the source tree.
 
 This script makes there be one place for forwarding to the correct platform's
@@ skipping 12 matching lines @@
   if not bin_path:
     print >> sys.stderr, ('gn.py: Could not find checkout in any parent of '
                           'the current path.\nThis must be run inside a '
                           'checkout.')
     return 1
   gn_path = os.path.join(bin_path, 'gn' + gclient_utils.GetExeSuffix())
   if not os.path.exists(gn_path):
     print >> sys.stderr, 'gn.py: Could not find gn executable at: %s' % gn_path
     return 2
   else:
-    return subprocess.call([gn_path] + sys.argv[1:])
+    return subprocess.call([gn_path] + args[1:])
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))