Update cssparser to fix correctness issue with serialization.

This issue was also exposed via CSS variables, and I've added a test for that
since it's how it was originally found in bug 1498188.

See w3c/csswg-drafts#4088 and servo/rust-cssparser#251

Differential Revision: https://phabricator.services.mozilla.com/D37148

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1563892
gecko-commit: d9f121b5943ecb4c917d54ea63824872bded0f38
gecko-integration-branch: central
gecko-reviewers: heycam

Author: emilio

css/css-variables/variable-substitution-basic.html

@@ -38,6 +38,12 @@
                 expectedValue:"0px 0px",
                 style:"--gap: 20;border-spacing: var(--gap)px;"
             },
+            {
+                testName:"You can't build up a single token where part of it is provided by a variable (percentages)",
+                propertyName:"text-indent",
+                expectedValue:"0px",
+                style:"--v: 20;text-indent: var(--v)%;"
+            },
             {
                 testName:"Multiple variable references in a single property",
                 propertyName:"border-spacing",
@@ -115,4 +121,4 @@
         });
     </script>
 </body>
-</html>
+</html>