letterboxing: backport bugzilla 1546832

https://bugzilla.mozilla.org/show_bug.cgi?id=1546832

Docked dev tools does not trigger letterboxing resize. Open TZP [1] on desktop and toggle the inspector on/off. Manually resize the inspector. The screen measurements are real-time.

Mozilla's patch also added the findbar, but that seems to already be applied in TB9 [Edit: need moar coffee: that needs fixing too and is in the patch]

[1] https://ghacksuserjs.github.io/TorZillaPrint/TorZillaPrint.html