Fix compiler warning on Win64

BUG=None
LOG=N
TBR=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/191153002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19731 ce2b1a6d-e550-0410-aec6-3dcde31c8c00