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

build/dir_exists.py

Index: build/dir_exists.py
diff --git a/build/dir_exists.py b/build/dir_exists.py
index 0a89bc87bbf6dab98c4d6d6aaf85848c96e4ca3d..70d367ec2690730f6318c4d8373750db5bb70fcd 100755
--- a/build/dir_exists.py
+++ b/build/dir_exists.py
@@ -8,8 +8,16 @@ 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())