Regression 5.15.8 -> 5.15.9 Renderer crash in certain Service Worker setups

There have been reports about renderer crashes with 5.15.9 on certain sites (e.g. https://www.cyclebristolctc.org.uk/rideinfo) that happen after visiting https://maps.google.com

We hit an assert due to a logic change in one of the patches in the 5.15.9 release.

Original report here https://github.com/qutebrowser/qutebrowser/issues/7118#issuecomment-1108983597