Reason for revert: Data race:
https://ci.chromium.org/p/v8/builders/ci/V8%20Linux64%20TSAN/34121

It was assume that MockPlatform runs everything on 1 thread. However,
MockPlatform::PostJob previously would schedule the job through
TestPlatform, which eventually posts concurrent tasks, thus causing
data race.
Fix: Manually calling NewDefaultJobHandle and passing the MockPlatform
ensures the jobs also run sequentially.

Additional change:
- CancelAndDetach is now called in ~CompilationStateImpl() to make sure
it's called in sequence with ScheduleCompileJobForNewUnits

Original CL description:
To avoid keeping around a list of job handles, CancelAndDetach() is
used in CancelCompilation. Dependency on WasmEngine is handled by a
barrier that waits on all jobs to finish.

Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2498659
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#71074}
Change-Id: Ie9556f7f96f6fb9a61ada0e5cbd58d4fb4a0f571
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2559137
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71459}