MicrotasksScope has accidentally ignored the given MicrotaskQueue instance
when it's scoping out. That confused the embedder to start using the non
default MicrotaskQueue.

Change-Id: Id345605cf6520cd073429b08698de75f7681d93c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1563836Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60765}