style: Hide -moz-gtk-buttonactivetext from content.

This prevents exposing the value to web content.

Differential Revision: https://phabricator.services.mozilla.com/D32611

components/style/values/specified/color.rs

@@ -172,6 +172,7 @@ pub enum SystemColor {
     MozOddtreerow,
 
     /// Used for button text when pressed.
+    #[parse(condition = "ParserContext::in_ua_or_chrome_sheet")]
     MozGtkButtonactivetext,
 
     /// Used for button text when pressed.
@@ -359,14 +360,12 @@ impl Parse for Color {
             Err(e) => {
                 #[cfg(feature = "gecko")]
                 {
-                    if let Ok(ident) = input.expect_ident() {
-                        if let Ok(system) = SystemColor::from_ident(ident) {
-                            return Ok(Color::System(system));
-                        }
-
-                        if let Ok(c) = gecko::SpecialColorKeyword::from_ident(ident) {
-                            return Ok(Color::Special(c));
-                        }
+                    if let Ok(system) = input.try(|i| SystemColor::parse(context, i)) {
+                        return Ok(Color::System(system));
+                    }
+
+                    if let Ok(c) = input.try(gecko::SpecialColorKeyword::parse) {
+                        return Ok(Color::Special(c));
                     }
                 }