style: Accept percentage for shape-image-threshold.

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

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.