Auto merge of #16340 - brainlessdeveloper:fix-border-radius-serialization, r=Wafflespeanut

Fix -moz-outline-radius shorthand serialization

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These changes aim to solve #15169 correcting the `ToCss` implementation for `LonghandsToSerialize` for the `border-radius` shorthands. They also reduce redundant values like `1px 2px 1px 2px` to `1px 2px` to either sides of the slash.

---
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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 #15169 (github issue number if applicable).

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- [x] There are tests for these changes

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components/style/properties/shorthand/outline.mako.rs

@@ -68,6 +68,7 @@
     for corner in ['topleft', 'topright', 'bottomright', 'bottomleft']
 )}" products="gecko" spec="Nonstandard (https://developer.mozilla.org/en-US/docs/Web/CSS/-moz-outline-radius)">
     use properties::shorthands;
+    use values::specified::basic_shape::serialize_radius_values;
 
     pub fn parse_value(context: &ParserContext, input: &mut Parser) -> Result<Longhands, ()> {
         // Re-use border-radius parsing.
@@ -80,19 +81,14 @@
         })
     }
 
-    // TODO: Border radius for the radius shorthand is not implemented correctly yet
     impl<'a> ToCss for LonghandsToSerialize<'a>  {
         fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-            try!(self._moz_outline_radius_topleft.to_css(dest));
-            try!(write!(dest, " "));
-
-            try!(self._moz_outline_radius_topright.to_css(dest));
-            try!(write!(dest, " "));
-
-            try!(self._moz_outline_radius_bottomright.to_css(dest));
-            try!(write!(dest, " "));
-
-            self._moz_outline_radius_bottomleft.to_css(dest)
+            serialize_radius_values(dest,
+                &self._moz_outline_radius_topleft.0,
+                &self._moz_outline_radius_topright.0,
+                &self._moz_outline_radius_bottomright.0,
+                &self._moz_outline_radius_bottomleft.0,
+            )
         }
     }
 </%helpers:shorthand>