style: Remove non-standard and unused system font values

And hide internal but used values. System fonts are not exposed in the
computed style so this should be fine.

If we need the old values for some obscure reason, it's trivial to alias
them to e.g., menu or so.

Differential Revision: https://phabricator.services.mozilla.com/D163269

components/style/properties/shorthands/font.mako.rs

@@ -71,7 +71,7 @@
         let mut stretch = None;
         let size;
         % if engine == "gecko":
-            if let Ok(sys) = input.try_parse(SystemFont::parse) {
+            if let Ok(sys) = input.try_parse(|i| SystemFont::parse(context, i)) {
                 return Ok(expanded! {
                      % for name in SYSTEM_FONT_LONGHANDS:
                         ${name}: ${name}::SpecifiedValue::system_font(sys),

components/style/values/specified/font.rs

@@ -72,21 +72,29 @@ macro_rules! system_font_methods {
 #[allow(missing_docs)]
 #[cfg(feature = "gecko")]
 pub enum SystemFont {
+    /// https://drafts.csswg.org/css-fonts/#valdef-font-caption
     Caption,
+    /// https://drafts.csswg.org/css-fonts/#valdef-font-icon
     Icon,
+    /// https://drafts.csswg.org/css-fonts/#valdef-font-menu
     Menu,
+    /// https://drafts.csswg.org/css-fonts/#valdef-font-message-box
     MessageBox,
+    /// https://drafts.csswg.org/css-fonts/#valdef-font-small-caption
     SmallCaption,
+    /// https://drafts.csswg.org/css-fonts/#valdef-font-status-bar
     StatusBar,
-    MozWindow,
+    /// Internal system font, used by the `<menupopup>`s on macOS.
-    MozDocument,
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
-    MozWorkspace,
-    MozDesktop,
-    MozInfo,
-    MozDialog,
-    MozButton,
     MozPullDownMenu,
+    /// Internal system font, used for `<button>` elements.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
+    MozButton,
+    /// Internal font, used by `<select>` elements.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     MozList,
+    /// Internal font, used by `<input>` elements.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     MozField,
     #[css(skip)]
     End, // Just for indexing purposes.