style: Make resolutions more like the rest of the CSS values.

Bug: 1460655
Reviewed-by: xidorn
MozReview-Commit-ID: 3Gt8VX1KhjC

components/style/values/computed/resolution.rs

@@ -0,0 +1,48 @@
+/* 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/. */
+
+//! Resolution values:
+//!
+//! https://drafts.csswg.org/css-values/#resolution
+
+use std::fmt::{self, Write};
+use style_traits::{CssWriter, ToCss};
+use values::computed::{Context, ToComputedValue};
+use values::specified;
+use values::CSSFloat;
+
+/// A computed `<resolution>`.
+pub struct Resolution(CSSFloat);
+
+impl Resolution {
+    /// Returns this resolution value as dppx.
+    #[inline]
+    pub fn dppx(&self) -> CSSFloat {
+        self.0
+    }
+}
+
+impl ToComputedValue for specified::Resolution {
+    type ComputedValue = Resolution;
+
+    #[inline]
+    fn to_computed_value(&self, _: &Context) -> Self::ComputedValue {
+        Resolution(self.to_dppx())
+    }
+
+    #[inline]
+    fn from_computed_value(computed: &Self::ComputedValue) -> Self {
+        specified::Resolution::Dppx(computed.dppx())
+    }
+}
+
+impl ToCss for Resolution {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: fmt::Write,
+    {
+        self.dppx().to_css(dest)?;
+        dest.write_str("dppx")
+    }
+}