gyp performance: don't invoke python to check dir existance

build/dir_exists.py

 #!/usr/bin/env python
 # Copyright (c) 2011 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.
 """Writes True if the argument is a directory."""
 
 import os.path
 import sys
 
 def main():
-  sys.stdout.write(str(os.path.isdir(sys.argv[1])))
+  sys.stdout.write(_is_dir(sys.argv[1]))
   return 0
 
+def _is_dir(dir_name):
+  return str(os.path.isdir(dir_name))
+
+def DoMain(args):
+  """Hook to be called from gyp without starting a separate python
+  interpreter."""
+  return _is_dir(args[0])
+
 if __name__ == '__main__':
   sys.exit(main())