Auto merge of #15662 - canaltinova:column, r=Manishearth

Fix parsing methods of column-{gap,width}

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They weren't accepting {normal, auto} keywords. Fixed parsing methods of these properties.

---
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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 #15088 (github issue number if applicable).

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- [X] There are tests for these changes

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components/style/values/specified/length.rs

@@ -496,10 +496,24 @@ impl Parse for Length {
     }
 }
 
-impl<T> Either<Length, T> {
+impl Either<Length, Normal> {
     #[inline]
     #[allow(missing_docs)]
-    pub fn parse_non_negative_length(_context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
+    pub fn parse_non_negative_length(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
+        if input.try(|input| Normal::parse(context, input)).is_ok() {
+            return Ok(Either::Second(Normal));
+        }
+        Length::parse_internal(input, AllowedNumericType::NonNegative).map(Either::First)
+    }
+}
+
+impl Either<Length, Auto> {
+    #[inline]
+    #[allow(missing_docs)]
+    pub fn parse_non_negative_length(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
+        if input.try(|input| Auto::parse(context, input)).is_ok() {
+            return Ok(Either::Second(Auto));
+        }
         Length::parse_internal(input, AllowedNumericType::NonNegative).map(Either::First)
     }
 }