to work around erroneous "illegal access" error on x64.

BUG=v8:2790
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/22877039

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16324 ce2b1a6d-e550-0410-aec6-3dcde31c8c00