Use smi zero instead of undefine_value to zap dead weak cells.

It is faster to test for smi zero from generated code.

BUG=
R=erikcorry@chromium.org

Review URL: https://codereview.chromium.org/637253004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24758 ce2b1a6d-e550-0410-aec6-3dcde31c8c00