Collecting all named grid areas in grid template shorthand

In this CL a bug related to grid areas shorthand serialization is fixed.
Previously, we just returned the first key of a named area in a given
row, thus losing named areas. This is fixed by collecting and appending
grid areas in a given row and column.

A test case in grid-template-shorthand-areas-valid.html is added to
validate the behavior.

Bug: 1303968
Change-Id: I7e91ba036250a0740e88a3f4e6fc6bbc95871d95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3514442
Reviewed-by: Daniel Libby <dlibby@microsoft.com>
Commit-Queue: Ana Sollano Kim <ansollan@microsoft.com>
Cr-Commit-Position: refs/heads/main@{#979954}

css/css-grid/parsing/grid-template-shorthand-areas-valid.html

@@ -30,6 +30,9 @@
 testValidGridTemplate("none / none", "\"a\"", "none");
 testValidGridTemplate("auto / 1px", "\"a a a\"", "\"a a a\" / 1px");
 testValidGridTemplate("auto / auto", "\"a a a\"", "\"a a a\" / auto");
+testValidGridTemplate("10px 20px 30px / 40px 50px 60px 70px",
+                      "\"a . b .\" \"c d . e\" \"f g h .\"",
+                      "\"a . b .\" 10px \"c d . e\" 20px \"f g h .\" 30px / 40px 50px 60px 70px");
 </script>
 </body>
 </html>