Fix for build issue on Mac introduced in r20948.

TBR=yangguo@chromium.org

Review URL: https://codereview.chromium.org/251363004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20951 ce2b1a6d-e550-0410-aec6-3dcde31c8c00