[dawn][native] Some cleanup of DeviceLostEvent code.

- Uses the refcount payload in RefCountedWithExternalCount to track
  whether an external ref count exists. Note that we are using a
  "ForTesting" function to implement the check of the count even
  though this is not for testing. We are special casing this because
  it is a valid check for the external ref count. This is needed to
  explicitly handle the spec behavior that the device lost event
  should pass nullptr for the device when the last external
  reference has been dropped.
- Makes APIGetLostFuture use cached future id when possible.
- Moves the device lost related members in Device to private since
  the mock isn't actually using the alternative constructor anymore.
  Also removes said constructor.
- Finally, defer cleanup of the callback infos to the completion of
  the device lost callback instead of having it called in two
  different places, the callback and in
  DeviceBase::WillDropLastExternalRef.

Change-Id: I31b57391438c0782ba223884a72991ca77426bec
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/299539
Commit-Queue: Loko Kung <lokokung@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>