interface is exposed through the global gc() function, we avoid 
being clever about forcing GCs when contexts are disposed and leave 
it to the embedder to make informed decisions about when to force 
a collection.
Review URL: http://codereview.chromium.org/40154

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1422 ce2b1a6d-e550-0410-aec6-3dcde31c8c00