Bug 1620467 - Part 6: Mark appearance values that are only used internally as chrome-only. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D83432

servo/components/style/values/specified/box.rs

@@ -1654,10 +1654,14 @@ pub enum Appearance {
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     ScrollbarbuttonRight,
     /// The scrollbar thumb.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     ScrollbarthumbHorizontal,
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     ScrollbarthumbVertical,
     /// The scrollbar track.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     ScrollbartrackHorizontal,
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     ScrollbartrackVertical,
     /// The scroll corner
     #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]