[devtools] Only cancel AsyncTaskContexts once

This CL is an attempt to fix a crash that is caused by DOMTimer
surviving past the owning v8::Isolate. The crash happens because
we try to resolve the ThreadDebugger instance from a stale v8::Isolate
pointer when deconstructing a DOMTimer (and an AsyncTaskContext in
return).

A DOMTimer explicitly calls `AsyncTaskContext::Cancel` when it is
stopped, including when the corresponding ExecutionContext is
destroyed. This means that very likely we attempt to cancel
the same AsyncTaskContext multiple times, which is unnecessary.

This CL sets the isolate_ pointer to nullptr after the first
cancel, this prevents further `Cancel` calls from accessing a
possible stale isolate_ pointer.

R=bmeurer@chromium.org

Bug: 1304066
Change-Id: Ie55c3e4138038c607ec477c0246b5f7de75fa473
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3512632
Commit-Queue: Simon Zünd <szuend@chromium.org>
Auto-Submit: Simon Zünd <szuend@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#979155}