This is not only dangerous (you might go down an infinite deref recursion if you hit a dead cycle) but entirely unnecessary (the subgraph is going to get deleted anyway regardless of ref counts).
Created attachment 145865 [details] the patch Still trying to figure out the best way to test this.
Landed in http://trac.webkit.org/changeset/119526
*** Bug 87994 has been marked as a duplicate of this bug. ***