Metal: implement ANGLE_device_metal extension. 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/+/2757810 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>