Fix output_dir argument to blink_idl_parser.py in BUILD.gn

Should be bindings_scripts_output_dir, not bindings_output_dir.

BUG=

Review URL: https://codereview.chromium.org/349413004

git-svn-id: svn://svn.chromium.org/blink/trunk@176841 bbb929c8-8fbe-4397-9dbb-9b2b20218538