Fix incorrect private access

(I'm puzzled why GCC did not report this error.)

TBR=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/669733007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24776 ce2b1a6d-e550-0410-aec6-3dcde31c8c00