Fix compilation error with latest Xcode.

The build failed because of unused private fields.

R=titzer@chromium.org
BUG=v8:3030
LOG=n

Review URL: https://codereview.chromium.org/94793003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18171 ce2b1a6d-e550-0410-aec6-3dcde31c8c00