Enable zapping of disposed global handles in release mode.

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

Review URL: https://codereview.chromium.org/12388080

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13818 ce2b1a6d-e550-0410-aec6-3dcde31c8c00