It's been removed from the spec:
https://github.com/gpuweb/gpuweb/pull/2127
Fixed: tint:1213
Change-Id: I163fe807765bb1ac0580b398f4897daea555216a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/67067
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>
Reviewed-by: James Price <jrprice@google.com>