Context::microtask_context can be null after v8::Context::DetachGlobal
is called, and that should cancel microtasks that are associated to
the detached context.
However, there are several callers left without the null check to the
microtask queue, and that causes crashes.

This CL adds the null check and cancellation as the crash fix.

Bug: chromium:937784
Change-Id: Ie8d107f28f200cee6e75798e3f72c5ed7a2a461c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1545139
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60623}