Fix overzealous casting that erroneously lead to ASSERTs

R=verwaest@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21372002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15991 ce2b1a6d-e550-0410-aec6-3dcde31c8c00