This is a band-aid for now, because I don't know how to best deal with
it.
Probably we should just print the plaintext selection contents,
unstyled, though that's a totally different codepath than what we do for
regular document selection, so probably deserves its own bug. Meanwhile,
this should prevent easy printing failures.
View-selection-source also doesn't deal with it properly so do the same
there too for now.
Differential Revision: https://phabricator.services.mozilla.com/D94595
b04577a9a7