Win64 fix for r19833.

R=dcarney@chromium.org
BUG=

Review URL: https://codereview.chromium.org/197083002

Patch from Daniel Vogelheim <vogelheim@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19837 ce2b1a6d-e550-0410-aec6-3dcde31c8c00