style: Don't waste an allocation when failing to parse a color.

I see that allocation show up in the profiles, and it makes sense, because
system colors and such are common in Firefox, and they're just wasting it.

Note that the clone() added is refcounted.

components/style/values/specified/color.rs

@@ -213,27 +213,33 @@ impl Color {
         // specified value.
         let start = input.state();
         let authored = match input.next() {
-            Ok(&Token::Ident(ref s)) => Some(s.to_lowercase().into_boxed_str()),
+            Ok(&Token::Ident(ref s)) => Some(s.clone()),
             _ => None,
         };
         input.reset(&start);
         match input.try(CSSParserColor::parse) {
             Ok(value) =>
                 Ok(match value {
                     CSSParserColor::CurrentColor => Color::CurrentColor,
-                    CSSParserColor::RGBA(rgba) => Color::Numeric {
-                        parsed: rgba,
-                        authored: authored,
-                    },
+                    CSSParserColor::RGBA(rgba) => {
+                        Color::Numeric {
+                            parsed: rgba,
+                            authored: authored.map(|s| s.to_lowercase().into_boxed_str()),
+                        }
+                    }
                 }),
             Err(e) => {
-                #[cfg(feature = "gecko")] {
+                #[cfg(feature = "gecko")]
+                {
                     if let Ok(system) = input.try(SystemColor::parse) {
                         return Ok(Color::System(system));
-                    } else if let Ok(c) = gecko::SpecialColorKeyword::parse(input) {
+                    }
+
+                    if let Ok(c) = gecko::SpecialColorKeyword::parse(input) {
                         return Ok(Color::Special(c));
                     }
                 }
+
                 match e {
                     BasicParseError { kind: BasicParseErrorKind::UnexpectedToken(t), location } => {
                         Err(location.new_custom_error(