[dawn][native] Stop using WaitListEvent in TrackedEvents.

- This change is a large refactoring to make TrackedEvents in Dawn
  native no longer backed by WaitListEvents. Previously, any
  TrackedEvent backed by a WaitListEvent implicitly creates a
  mutex. This also meant that when we go to wait on these events,
  we would need to go through the complicated logic in
  WaitListEvent::WaitAny to acquire the relevant locks. This
  change, however, exploits the fact that event state can only
  be changed via the EventManager which already has a single
  lock protecting the events. As such, we can fold in a lot of
  the logic in WaitListEvent into the EventManager directly
  and avoid creating O(N) locks per event.
- This change also updates WaitAny to use spans where relevant,
  and cleans up all the code that becomes obsolete with the
  refactoring specified above.

Change-Id: Id9a714db626ec9b08aa731b173e9f525dfe7d40b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/300818
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>