Auto merge of #15357 - dashed:gh-15207, r=Wafflespeanut,emilio

Refactor outline-style to accept "auto" value in addition to border-style values.

Fixes https://github.com/servo/servo/issues/15207

Refactored as per https://github.com/servo/servo/issues/15207#issuecomment-275171590 .

<!-- Please describe your changes on the following line: -->

- [x] Correct refactor? I'd appreciate any feedback on this.
- [x] ~~~proper borderstyle value for `outline-style: auto;`?~~~ ~~~(EDIT: deferred to a `FIXME`)~~~ (EDIT2: it is now solid for behaviour parity with firefox)
- [x] squash pending PR review
- [x] mako code review

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15207 (github issue number if applicable).

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- [x] There are tests for these changes

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components/style/values/mod.rs

@@ -25,9 +25,9 @@ macro_rules! define_numbered_css_keyword_enum {
             $( $variant = $value ),+
         }
 
-        impl $name {
+        impl Parse for $name {
             #[allow(missing_docs)]
-            pub fn parse(input: &mut ::cssparser::Parser) -> Result<$name, ()> {
+            fn parse(_context: &ParserContext, input: &mut ::cssparser::Parser) -> Result<$name, ()> {
                 match_ignore_ascii_case! { try!(input.expect_ident()),
                     $( $css => Ok($name::$variant), )+
                     _ => Err(())