Fix size_t to int conversion compile error.

BUG=

TBR=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/552313002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23805 ce2b1a6d-e550-0410-aec6-3dcde31c8c00