style: Add a more sensible scrollbar size API to nsITheme, and use it to replace ScrollbarNonDisappearing.

There's no reason we should need an scrollbar box to query the size of a
scrollbar. I plan to use this in the following patch to make the size of a
resizer not vary depending on whether the container has scrollbars or not,
which is what ultimately causes the reftest failure.

Differential Revision: https://phabricator.services.mozilla.com/D103302

components/style/values/specified/box.rs

@@ -1834,10 +1834,6 @@ pub enum Appearance {
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     MozMacVibrantTitlebarLight,
 
-    /// A non-disappearing scrollbar.
-    #[css(skip)]
-    ScrollbarNonDisappearing,
-
     /// A themed focus outline (for outline:auto).
     ///
     /// This isn't exposed to CSS at all, just here for convenience.