Implement a URL-generic type for list-style-image

This should fix the following two "expected to fail" tests:

- getComputedStyle(elem) for url() listStyleImage uses the resolved URL
  and elem.style uses the original URL

- getComputedStyle(elem) for url() listStyle uses the resolved URL
  and elem.style uses the original URL

components/style/values/generics/url.rs

@@ -0,0 +1,39 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Generic types for url properties.
+
+use cssparser::Parser;
+use parser::{Parse, ParserContext};
+use style_traits::ParseError;
+
+/// An image url or none, used for example in list-style-image
+#[derive(Animate, Clone, ComputeSquaredDistance, Debug, MallocSizeOf)]
+#[derive(PartialEq, ToAnimatedValue, ToAnimatedZero, ToComputedValue, ToCss)]
+pub enum UrlOrNone<Url> {
+    /// `none`
+    None,
+    /// `A URL`
+    Url(Url),
+}
+
+impl<Url> UrlOrNone<Url> {
+    /// Initial "none" value for properties such as `list-style-image`
+    pub fn none() -> Self {
+        UrlOrNone::None
+    }
+}
+
+impl<Url> Parse for UrlOrNone<Url> where Url: Parse {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<UrlOrNone<Url>, ParseError<'i>> {
+        if let Ok(url) = input.try(|input| Url::parse(context, input)) {
+            return Ok(UrlOrNone::Url(url));
+        }
+        input.expect_ident_matching("none")?;
+        Ok(UrlOrNone::None)
+    }
+}