Allow user to query internal MTLDevice used by Metal back-end.

Bug: angleproject:2634
Change-Id: I9b0a6eaad8c634069f43e47d5f4f88f61da35f74
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2757810Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>