Fix for issue 3071: A zombie AlloctionSite must clear pointer fields.

BUG=v8:3071
R=yangguo@chromium.org
LOG=N

Review URL: https://codereview.chromium.org/125873002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18469 ce2b1a6d-e550-0410-aec6-3dcde31c8c00