This removes the capability of having differently-sized vertical and
horizontal scrollbars (which is only potentially used in windows, and in
practice almost-never used). For that case, we choose the larger of
vertical/horizontal scrollbar sizes.

This is in order to be able to realistically expose the scrollbar size
to CSS, see blocked bug.

We make RecomputeScrollbarParams the central place where each scrollbar
style decides its sizes, and make GetDPIRatioForScrollbarPart handle the
cocoa special-case of scaling to 1 or 2, but nothing else.

Differential Revision: https://phabricator.services.mozilla.com/D168080