The callers to NumberToSize are supposed to validate the number, but
this adds a last line of defense.

TBR=jkummerow@chromium.org, ulan@chromium.org

Review URL: https://codereview.chromium.org/61733021

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17737 ce2b1a6d-e550-0410-aec6-3dcde31c8c00