style: Improve #[derive(Parse)].

I want to do this so that I can get rid of Either<>. The reasons for getting rid
of either are multiple:

 * It doesn't generate as nice C++ code using cbindgen.
 * It isn't that nice to use either from Rust.
 * cbindgen has bugs with zero-sized types.

I started using this for ColorOrAuto and a few others, for now.

Differential Revision: https://phabricator.services.mozilla.com/D19844

components/style/values/specified/flex.rs

@@ -4,29 +4,12 @@
 
 //! Specified types for CSS values related to flexbox.
 
-use crate::parser::{Parse, ParserContext};
 use crate::values::generics::flex::FlexBasis as GenericFlexBasis;
 use crate::values::specified::Size;
-use cssparser::Parser;
-use style_traits::ParseError;
 
 /// A specified value for the `flex-basis` property.
 pub type FlexBasis = GenericFlexBasis<Size>;
 
-impl Parse for FlexBasis {
-    fn parse<'i, 't>(
-        context: &ParserContext,
-        input: &mut Parser<'i, 't>,
-    ) -> Result<Self, ParseError<'i>> {
-        if let Ok(size) = input.try(|i| Size::parse(context, i)) {
-            return Ok(GenericFlexBasis::Size(size));
-        }
-        try_match_ident_ignore_ascii_case! { input,
-            "content" => Ok(GenericFlexBasis::Content),
-        }
-    }
-}
-
 impl FlexBasis {
     /// `auto`
     #[inline]