style: Merge the two scrollbar color properties into scrollbar-color.

Differential Revision: https://phabricator.services.mozilla.com/D6115

components/style/values/computed/ui.rs

@@ -20,3 +20,6 @@ pub type Cursor = generics::Cursor<CursorImage>;
 
 /// A computed value for item of `image cursors`.
 pub type CursorImage = generics::CursorImage<ComputedImageUrl, Number>;
+
+/// A computed value for `scrollbar-color` property.
+pub type ScrollbarColor = generics::ScrollbarColor<Color>;

components/style/values/generics/ui.rs

@@ -67,3 +67,27 @@ impl<ImageUrl: ToCss, Number: ToCss> ToCss for CursorImage<ImageUrl, Number> {
         Ok(())
     }
 }
+
+/// A generic value for `scrollbar-color` property.
+///
+/// https://drafts.csswg.org/css-scrollbars-1/#scrollbar-color
+#[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug, MallocSizeOf, PartialEq,
+         SpecifiedValueInfo, ToAnimatedValue, ToAnimatedZero, ToComputedValue, ToCss)]
+pub enum ScrollbarColor<Color> {
+    /// `auto`
+    Auto,
+    /// `<color>{2}`
+    Colors {
+        /// First `<color>`, for color of the scrollbar thumb.
+        thumb: Color,
+        /// Second `<color>`, for color of the scrollbar track.
+        track: Color,
+    }
+}
+
+impl<Color> Default for ScrollbarColor<Color> {
+    #[inline]
+    fn default() -> Self {
+        ScrollbarColor::Auto
+    }
+}

components/style/values/specified/ui.rs

@@ -122,3 +122,21 @@ impl From<MozForceBrokenImageIcon> for u8 {
         }
     }
 }
+
+/// A specified value for `scrollbar-color` property
+pub type ScrollbarColor = generics::ScrollbarColor<Color>;
+
+impl Parse for ScrollbarColor {
+    fn parse<'i, 't>(
+        context: &ParserContext,
+        input: &mut Parser<'i, 't>,
+    ) -> Result<Self, ParseError<'i>> {
+        if input.try(|i| i.expect_ident_matching("auto")).is_ok() {
+            return Ok(generics::ScrollbarColor::Auto);
+        }
+        Ok(generics::ScrollbarColor::Colors {
+            thumb: Color::parse(context, input)?,
+            track: Color::parse(context, input)?,
+        })
+    }
+}