Custom platform implementations which do not run all foreground tasks
before shutting down an isolate could trigger a DCHECK (see linked
issue).
This was introduced in https://crrev.com/c/2578980, but the bug does not
trigger in the default platform implementation, hence went unnoticed.

Since it's only a spurious DCHECK error without security implications, I
decided against a test with a custom platform implementation to
reproduce the bug.

R=ahaas@chromium.org

Bug: v8:11456
Change-Id: Ie01562af7bdffe23cad7172180f2bc19ea61dfda
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2697189Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72774}