Fix Win64 size_t/int conversion after b3f2277ea42a.

TBR=dcarney@chromium.org

Review URL: https://codereview.chromium.org/646983005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24720 ce2b1a6d-e550-0410-aec6-3dcde31c8c00