This code is super-hairy, but I think this is the minimum amount of changes
that we need.
nsPlainTextSerializer::IsInPre() before this patch is completely broken, and
I changed it to maintain a stack of bools representing whether the elements
that we saw as we were traversing the tree are preformatted or not.
nsXHTMLContentSerializer maintains this information using a counter, which is
broken in case pre and non-preformatted elements are stacked underneath each
other, but I'm not sure why this code is using a counter and I didn't want to
change it drastically, so for now I'm just making it look at the element's
style first as opposed to its tag name.
Follow-up work may include exploring whether nsXHTMLContentSerializer should
use a stack similar to nsPlainTextSerializer, and also audit this code for
more places where things are hardcoded based on tag names where we should be
really looking at the style.
