it from the old heuristic (disabled if the new notications are 
used) and make sure to not force a GC if one has already happened
after V8 received the last context disposal notification.
Review URL: http://codereview.chromium.org/660267

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3984 ce2b1a6d-e550-0410-aec6-3dcde31c8c00