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 .

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- [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

---
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- [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/layout/display_list_builder.rs

@@ -1072,15 +1072,18 @@ impl FragmentDisplayListBuilding for Fragment {
                                                     style: &ServoComputedValues,
                                                     bounds: &Rect<Au>,
                                                     clip: &ClippingRegion) {
+        use style::values::Either;
+
         let width = style.get_outline().outline_width;
         if width == Au(0) {
             return
         }
 
-        let outline_style = style.get_outline().outline_style;
-        if outline_style == border_style::T::none {
-            return
-        }
+        let outline_style = match style.get_outline().outline_style {
+            Either::First(_auto) => border_style::T::solid,
+            Either::Second(border_style::T::none) => return,
+            Either::Second(border_style) => border_style
+        };
 
         // Outlines are not accounted for in the dimensions of the border box, so adjust the
         // absolute bounds.