style: Move represents_keyword to the css attributes.

Bug: 1457635
Reviewed-by: xidorn
MozReview-Commit-ID: 21yuU4h34AQ

components/style/values/specified/position.rs

@@ -424,14 +424,14 @@ pub enum AutoFlow {
 }
 
 #[derive(Clone, Copy, Debug, Eq, MallocSizeOf, PartialEq, SpecifiedValueInfo,
-         ToComputedValue)]
+         ToComputedValue, ToCss)]
 /// Controls how the auto-placement algorithm works
 /// specifying exactly how auto-placed items get flowed into the grid
 pub struct GridAutoFlow {
     /// Specifiy how auto-placement algorithm fills each `row` or `column` in turn
     pub autoflow: AutoFlow,
     /// Specify use `dense` packing algorithm or not
-    #[value_info(represents_keyword)]
+    #[css(represents_keyword)]
     pub dense: bool,
 }
 
@@ -446,20 +446,6 @@ impl GridAutoFlow {
     }
 }
 
-impl ToCss for GridAutoFlow {
-    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
-    where
-        W: Write,
-    {
-        self.autoflow.to_css(dest)?;
-
-        if self.dense {
-            dest.write_str(" dense")?;
-        }
-        Ok(())
-    }
-}
-
 impl Parse for GridAutoFlow {
     /// [ row | column ] || dense
     fn parse<'i, 't>(