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/gecko/values.rs

@@ -11,7 +11,7 @@ use cssparser::RGBA;
 use gecko_bindings::structs::{nsStyleCoord, StyleGridTrackBreadth, StyleShapeRadius};
 use gecko_bindings::sugar::ns_style_coord::{CoordData, CoordDataMut, CoordDataValue};
 use std::cmp::max;
-use values::{Auto, Either, None_};
+use values::{Auto, Either, None_, Normal};
 use values::computed::{Angle, LengthOrPercentageOrNone, Number};
 use values::computed::{LengthOrPercentage, LengthOrPercentageOrAuto};
 use values::computed::basic_shape::ShapeRadius;
@@ -257,6 +257,20 @@ impl GeckoStyleCoordConvertible for None_ {
     }
 }
 
+impl GeckoStyleCoordConvertible for Normal {
+    fn to_gecko_style_coord<T: CoordDataMut>(&self, coord: &mut T) {
+        coord.set_value(CoordDataValue::Normal)
+    }
+
+    fn from_gecko_style_coord<T: CoordData>(coord: &T) -> Option<Self> {
+        if let CoordDataValue::Normal = coord.as_value() {
+            Some(Normal)
+        } else {
+            None
+        }
+    }
+}
+
 /// Convert a given RGBA value to `nscolor`.
 pub fn convert_rgba_to_nscolor(rgba: &RGBA) -> u32 {
     ((rgba.alpha as u32) << 24) |