This avoids an unnecessary runtime call from Set.prototype.delete().

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/290733008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21408 ce2b1a6d-e550-0410-aec6-3dcde31c8c00