Use ARRAY_SIZE instead of DIY code.

This improves r22769.

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/437153002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22804 ce2b1a6d-e550-0410-aec6-3dcde31c8c00