https://github.com/w3c/web-platform-tests/pull/7677 was created for https://chromium-review.googlesource.com/710504 but the final CL didn't have any exportable changes. (They were dropped in PS5, "Fix a whoops".)

It's possible that  issue 774781  is the reason, but maybe we just don't have the logic to close PRs where there's nothing to export in the end.