Everyone calls them with the shell of the current composed document, and this
allows the multi-presShell stuff to just be in UpdateCurrentStyleSources /
DoGetStyleContextNoFlush.
The only reason we need to use OwnerDoc()->GetShell() instead of the composed
doc in GetStyleContext / GetStyleContextNoFlush is Element::GetBindingURL, which
does expect to get the binding URL for stuff outside of the composed doc (and
changing that gave me a useless browser).
That's technically a behavior change on the cases that used to pass nullptr, but
I think all callers are fine with that. I could also just add a special function
for that particular case, it may be worth it.
MozReview-Commit-ID: 2XlnkgdgDCK