It seems to be that we had taken the wrong assumption that there are only one window with one page, therefore there are two ownership related problem that breaks MiniBrowser when there are multiply windows and the user closing one of them.
Created attachment 67175 [details] proposed patch
Comment on attachment 67175 [details] proposed patch LGTM
Comment on attachment 67175 [details] proposed patch Clearing flags on attachment: 67175 Committed r67266: <http://trac.webkit.org/changeset/67266>
All reviewed patches have been landed. Closing bug.