This avoids grabbing the document when values that inherit from the body,
whatever that means, aren't under the body.
In that case we'll get a semi-random value, but that's also mishandled by Gecko
anyways (and probably Blink, though haven't tested), and doesn't really make
much sense.
The only reason why we had the `existing_style_for_style_damage` bit is to apply
some optimizations that we don't have anymore.
I still want to reintroduce a few of them, at least for the non-eager
pseudo-element case... But I think I won't need this at all.
This allows us to remove a fair amount of Gecko code too.
Deduplicating code is nice, and it will help us when we make the bit
propagation more complicated in upcoming patches.
MozReview-Commit-ID: KIQnNJVayrM
The alias is left there temporarilly and will be removed completely in a later commit where
also components/style/gecko/generated/structs_{debug|release}.rs are re-generated (they still
use the old alias).
This allows us to have ensure_data() and clear_data() functions on the TElement
trait, instead of hacking around it adding methods in random traits.
This also allows us to do some further cleanup, which I'd rather do in a
followup.
