Make V8 compile with Win64 dbg

BUG=chromium:420538
R=dcarney@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/634493002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24408 ce2b1a6d-e550-0410-aec6-3dcde31c8c00