Fix Win64 compilation after r14670

TBR=jkummerow@chromium.org
BUG=v8:2628

Review URL: https://codereview.chromium.org/15133007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14671 ce2b1a6d-e550-0410-aec6-3dcde31c8c00