[dawn][native] Make EventManager::SetFutureReady take Ref<TrackedEvent>.

- Before, because EventManager::SetFutureReady took a TrackedEvent*,
  TSAN found that another thread could race on a WaitAny, and as soon
  as TrackedEvent::SetReadyToComplete was called, the waiting thread
  could complete the event instead of the thread that called
  SetFutureReady. Usually, this isn't a problem because the pointer
  that is passed to the function is backed by a Ref owned by the
  caller, however, in DeviceLost, part of the Complete function
  actually nulls out the Ref that the caller is expecting to exist
  to keep the Event alive. As a result, back in SetFutureReady,
  where we continue to use the pointer, we can end up using a
  destroyed TrackedEvent object.
- This change ensures that now, an additional reference of the event
  exists and keeps the pointer alive for the duration of
  SetFutureReady.

Bug: 491515775
Change-Id: I996901da9b3306fe7fbdbbf33afc562f3518f7b1
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/298635
Auto-Submit: Loko Kung <lokokung@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>