Zero the workgroup memory for all backends.
We can probably disable this for the backends that support workgroup zeroing, but that's an optimization we can perform later.
Fixed: tint:280
Change-Id: I9cad919ba3a15b8cedfe6939317d1f6b95425453
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/55244
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Reviewed-by: James Price <jrprice@google.com>
75db82c96b