servo: Merge #19581 - style: Fix grid line serialization to avoid redundant spacing (from emilio:grid-line-serialize); r=canaltinova

This fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1425227.

Source-Repo: https://github.com/servo/servo
Source-Revision: a31fd24ce93f3a8bde1b56e01109ee3b94532e0d

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : ad4c63f930027b6a896a3439e2af453140b63db7

servo/components/style/values/generics/grid.rs

@@ -59,12 +59,16 @@ where
         }
 
         if let Some(ref i) = self.line_num {
-            dest.write_str(" ")?;
+            if self.is_span {
+                dest.write_str(" ")?;
+            }
             i.to_css(dest)?;
         }
 
         if let Some(ref s) = self.ident {
-            dest.write_str(" ")?;
+            if self.is_span || self.line_num.is_some() {
+                dest.write_str(" ")?;
+            }
             s.to_css(dest)?;
         }