They're not valid, since the embedder is allowed to process
tasks in several threads, if they do it in a thread safe manner.

Bug: v8:10239
Change-Id: I6c397a8bba75ab7aec3ee8ea8de416af817d9514
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2342846Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69285}