Fortunately this is not an API break because it was actually unused, if anybody is using a boolean callback it will still work. I guess it's another copy-paste error :-(
Created attachment 220820 [details] Patch
Thanks for the patch. If this patch contains new public API please make sure it follows the guidelines for new WebKit2 GTK+ API. See http://trac.webkit.org/wiki/WebKitGTK/AddingNewWebKit2API
Committed r161805: <http://trac.webkit.org/changeset/161805>