MicrotaskQueue associated to Context may be null after DetachGlobal,
and triggering FinalizationGroup clean up on the detached context
causes a crash.
This CL fixes the crash by cancelling the clean up on such a context.

Bug: chromium:937784
Change-Id: I57883ae0caf6c6bb35e482e441b6e09e921d9def
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1552500Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60931}