[native] Use a toggle instead of feature for spontaneous queue events.

- This change reverts commit 2143462ffbf02b546100b966acb7c54580f958b9,
  (more details below) and instead makes the feature a toggle instead.
  This is necessary because features require explicit user enabling,
  whereas spontaneous queue events is really just an implementation
  detail in Dawn.
- The reverted change was also causing many of the event tests that
  were previously enabled to have become unintentionally disabled
  because the test device wasn't created with the feature passed,
  even though Metal always has spontaneous queues now. This change
  fixes that and verifies that those tests are running again.

Original change's description:
> [dawn][native] Add an explicit feature for spontaneous queue events.
>
> - I probably should have added this earlier, but this makes it a bit
>   easier to gate tests and other features that depend on this.
>
> Bug: 412761635
> Change-Id: I95b0ee59a30ae1f91d0081a83338b99bb8866f3c
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/299776
> Reviewed-by: Kai Ninomiya <kainino@chromium.org>
> Commit-Queue: Loko Kung <lokokung@google.com>

Bug: 412761635
Change-Id: Ic36d2599876e2bdd295b7f3ff63f6e734ee0e557
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/303515
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>