Revert "Reland "Refactor AsyncTask into an object with helpers.""

This reverts commit 42a063536b5e17c4ed79b65043facf8e2eca7793.

Reason for revert: Still fails on linux-clang-dbg-x86

Original change's description:
> Reland "Refactor AsyncTask into an object with helpers."
>
> This is a reland of commit 773146ed837bac5363753246e4e2ea3f476f20b1
>
> Original change's description:
> > Refactor AsyncTask into an object with helpers.
> >
> > AsyncTaskManager::PostTask now constructs a task object to wrap the
> > running task and returns it. Allow for overriding the AsyncTask class
> > with a template on PostTask.
> >
> > Update std::lock_guard to the more modern std::scoped_lock in
> > AsyncTaskManager.
> >
> > Bug: 406520956
> >
> > Change-Id: Ie66e5b704db3772117885ce71ea0308b13c093b7
> > Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/251114
> > Reviewed-by: Corentin Wallez <cwallez@chromium.org>
> > Reviewed-by: Loko Kung <lokokung@google.com>
> > Commit-Queue: Geoff Lang <geofflang@chromium.org>
>
> Bug: 406520956
> Change-Id: Ifdeef6a5ed973a58e5826e20e9b371ba5c5ac2d0
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/261096
> Reviewed-by: Loko Kung <lokokung@google.com>
> Reviewed-by: Corentin Wallez <cwallez@chromium.org>
> Commit-Queue: Geoff Lang <geofflang@chromium.org>

TBR=cwallez@chromium.org,geofflang@chromium.org,dawn-scoped@luci-project-accounts.iam.gserviceaccount.com,lokokung@google.com

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 406520956
Change-Id: Ifaeb824fb1fb18a3707325050a23f321deacd52e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/261335
Reviewed-by: Loko Kung <lokokung@google.com>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>