Reland "[dawn][metal] Makes AllowSpontaneous work for Metal queue events."

Relanding now that:
  - *MappedRange functions no longer acquire the device-wide lock.
  - The |completedSerial| in QueueAndSerial is atomic.

Previously, the issues above along with some lock scopes resulted in
deadlocks when spontaneous map async callbacks fired.

Original change's description:
> Revert "[dawn][metal] Makes AllowSpontaneous work for Metal queue events."
>
> This reverts commit b1c66af72925e24732b7c0f7c7275714d960b842.
>
> Reason for revert: Failures on Intel and AMD Macs, e.g.:
> https://ci.chromium.org/ui/p/chromium/builders/ci/Dawn%20Mac%20x64%20DEPS%20Release%20%28AMD%29
>
> Bug: 412761367
> Original change's description:
> > [dawn][metal] Makes AllowSpontaneous work for Metal queue events.
> >
> > - Since the queues in Metal are updated via Metal callbacks already,
> >   we can make AllowSpontaneous work by having the Metal callbacks
> >   directly call the queue callbacks as well.
> > - Adds spontaneous tests for queue events in Metal.
> >
> > Bug: 412761367
> > Change-Id: I83b338455c8271ed680faa0718ead2c9e66388be
> > Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/236935
> > Reviewed-by: Kai Ninomiya <kainino@chromium.org>
> > Commit-Queue: Loko Kung <lokokung@google.com>
>
> Bug: 412761367
> Change-Id: Ib6da7715e840afa7b6126c2f346385d22200f56a
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/242055
> Reviewed-by: Loko Kung <lokokung@google.com>
> Commit-Queue: Kai Ninomiya <kainino@chromium.org>
> Reviewed-by: Kai Ninomiya <kainino@chromium.org>

Bug: 412761367
Cq-Include-Trybots: luci.chromium.try:dawn-try-mac-amd-exp,dawn-try-mac-arm64-m2-exp,dawn-try-mac-intel-exp,mac-arm64-dawn-rel,mac-dawn-rel
Include-Ci-Only-Tests: true
Change-Id: I203a3c82225c27e486b16fcca610379e21e8f4a3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/242334
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>