style: Accept percentage for shape-image-threshold.

Differential Revision: https://phabricator.services.mozilla.com/D39241

components/style/properties/longhands/box.mako.rs

@@ -649,9 +649,10 @@ ${helpers.predefined_type(
 
 // The spec issue for the parse_method: https://github.com/w3c/csswg-drafts/issues/4102.
 ${helpers.predefined_type(
-    "shape-image-threshold", "Opacity", "0.0",
+    "shape-image-threshold",
+    "Opacity",
+    "0.0",
     engines="gecko",
-    parse_method="parse_number",
     animation_value_type="ComputedValue",
     spec="https://drafts.csswg.org/css-shapes/#shape-image-threshold-property",
 )}

components/style/values/specified/mod.rs

@@ -410,17 +410,6 @@ impl Parse for NonNegativeNumberOrPercentage {
 )]
 pub struct Opacity(Number);
 
-impl Opacity {
-    /// Parse number value only.
-    #[inline]
-    pub fn parse_number<'i, 't>(
-        context: &ParserContext,
-        input: &mut Parser<'i, 't>,
-    ) -> Result<Self, ParseError<'i>> {
-        Number::parse(context, input).map(Opacity)
-    }
-}
-
 impl Parse for Opacity {
     /// Opacity accepts <number> | <percentage>, so we parse it as NumberOrPercentage,
     /// and then convert into an Number if it's a Percentage.