Implement Element::has_css_layout_box() and related methods by inspecting the computed style

[jonleighton]

This is a follow-up to #19803 suggested by @emilio.

Yeah, this should be enough I think... I was expecting a query that returned an Option<Arc<ComputedValues>>, and we'd just need to check the display value (because nodes under display: none subtrees don't have any style).

That would be useful on its own to implement the other overflow_x / y and other style queries in a fast way, but this also works :)

I'll look into it once that PR is merged.

[jonleighton]

@emilio I just had a preliminary look at what this involves, and noticed that there is already a margin_style query which returns a subset of the computed styles. Do you think it would make sense to replace that with a more generic one which returns all the computed styles? Is there a danger of copying around too much data here?

[emilio]

Yes, I think it makes sense to remove margin_style.

And no, there's not much danger of copying too much data. Styles are stored as an Arc<ComputedValues>, so returning the style is literally just a refcount bump (an arc clone)