If object enters NEAR_DEATH state, it must be explicitly cleared and/or disposed, otherwise
it would retain JS object forever.  Note as well that parameter is reset to NULL on first
invocation so weak handle callback would be in hard situation.

Review URL: http://codereview.chromium.org/3011009

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5096 ce2b1a6d-e550-0410-aec6-3dcde31c8c00