As seen in bug 119915, caching failure results in an assertion here. This can happen in normal browsing too, not only in WKTR.
Created attachment 209123 [details] proposed fix It's not super clear to me whether it's intentional or a mistake that this variable is not set for subframes. The only other place where it is used is HistoryController::restoreDocumentState(), and I couldn't make a test case that would show any effect this patch could have on form value restoration. Hopefully, there is no effect.
Created attachment 209130 [details] also delete unneeded code
Comment on attachment 209130 [details] also delete unneeded code Clearing flags on attachment: 209130 Committed r154306: <http://trac.webkit.org/changeset/154306>
All reviewed patches have been landed. Closing bug.
The test added by this patch is failing on GTK+, Qt, and EFL bots: http://webkit-test-results.appspot.com/dashboards/flakiness_dashboard.html#tests=http%2Ftests%2Fnavigation%2Fpost-frames-goback1-uncached.html e.g. http://build.webkit.org/results/GTK%20Linux%2064-bit%20Debug%20WK1/r154378%20(3715)/results.html