There's a possible reference cycle between Document and its form elements if the Document gets detached from its frame before the timer started in didAssociateFormControls() fires.
Created attachment 307384 [details] Patch
Comment on attachment 307384 [details] Patch Clearing flags on attachment: 307384 Committed r215465: <http://trac.webkit.org/changeset/215465>
All reviewed patches have been landed. Closing bug.