[dawn][wire] Ensure that Devices on the Server always call Destroy.

- Because the wire server creates and manages callback information
  for all Devices, we need to ensure that all Devices' callbacks
  are fired before the server goes away. Otherwise, if the server
  is deleted, and somehow there is an outstanding reference to the
  backing native Device, the callbacks can happen and reference
  freed memory.
- Updates Wire testing infrastructure to:
  1) Use a NiceMock for the ProcTable to avoid overly strict tests
     that end up adding a lot implementation detail expectations.
  2) Add expecatations that successfully created Devices on the
     server should call Destroy to ensure that their callbacks are
     all flushed.
  3) Remove some tech-debt/unused members and functions after the
     change.

Bug: 491518608
Change-Id: I136f7c94ee7e2d79b5b04796bf850a990300aba4
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/297136
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>