It was experimental implementation, since then the v8 is used in a single heap - single thread mode and the code in DOMData that was delaying removal of the objects if GC happens not on their native thread is not needed.
Created attachment 50566 [details] Patch. I've run this through Chromium try bots, all green.
landed: http://trac.webkit.org/changeset/55882
It's always nice to see that many lines killed---thanks a lot, Dmitry.