callbacks.

Otherwise if callback allocates a new handle, it could orphan some global handles (with disastorous
consequences if those global handles are cached).

Review URL: http://codereview.chromium.org/395024

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3318 ce2b1a6d-e550-0410-aec6-3dcde31c8c00