merge_pr_10

Fix grid-area-valid.html test to not accept serialization results that are clearly invalid.

w3c/csswg-drafts#2858

Differential Revision: https://phabricator.services.mozilla.com/D54417

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1598932
gecko-commit: fec93f8968c976eb4c84f2bcd62df2a95e004a75
gecko-integration-branch: autoland
gecko-reviewers: emilio