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

This is a reland of commit a2bc4fa3d9a37ff02bef22ab81a42d7cd7c34bc2

Original change's description:
> [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>

Change-Id: I32944d1e762bf425531c5b989c9757dde2a7df1d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/301036
Commit-Queue: Loko Kung <lokokung@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>