Fix Math.round/floor that had bogus Smi representation

BUG=chromium:272564
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/23022005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16185 ce2b1a6d-e550-0410-aec6-3dcde31c8c00