style: When resetting background color for high contrast, preserve alpha channel appropriately.

But discard it when backplating behind text, so that text is readable.

This should be uncontroversial... Dealing with widgets is a bit harder
so TBD.

Differential Revision: https://phabricator.services.mozilla.com/D91779

components/style/values/specified/color.rs

@@ -529,15 +529,6 @@ impl Color {
         parse_hash_color(&serialization)
             .map_err(|()| location.new_custom_error(StyleParseErrorKind::UnspecifiedError))
     }
-
-    /// Returns true if the color is completely transparent, and false
-    /// otherwise.
-    pub fn is_transparent(&self) -> bool {
-        match *self {
-            Color::Numeric { ref parsed, .. } => parsed.alpha == 0,
-            _ => false,
-        }
-    }
 }
 
 #[cfg(feature = "gecko")]