Make shorthand serializer for "column-rule"

Make function GetShorthandValueForColumnRule to omit initial values
during shorthand serialization

Bug: 1429866
Change-Id: Ief5c04aed4c24c4f8d8412a978755d08d1cd89b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4546749
Reviewed-by: Anders Hartvoll Ruud <[email protected]>
Commit-Queue: Hyomin Kim <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1147743}

css/css-multicol/parsing/column-rule-valid.html

@@ -16,6 +16,7 @@
 test_valid_value("column-rule", "red");
 
 test_valid_value("column-rule", "currentcolor hidden medium", "hidden");
+test_valid_value("column-rule", "currentcolor none medium", "medium");
 </script>
 </body>
 </html>