Add zapping of disposed persistent handles.

R=svenpanne@chromium.org
BUG=chromium:176056

Review URL: https://codereview.chromium.org/12260060

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13675 ce2b1a6d-e550-0410-aec6-3dcde31c8c00