validation: compute shader must include 'workgroup_size' in its attributes

Bug: tint:884
Change-Id: If96c6df3247fee142a779117fa26d006afd4f7ef
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/55680
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/test/ptr_ref/load/local/ptr_private.wgsl.expected.wgsl b/test/ptr_ref/load/local/ptr_private.wgsl.expected.wgsl
index a865378..4c92811 100644
--- a/test/ptr_ref/load/local/ptr_private.wgsl.expected.wgsl
+++ b/test/ptr_ref/load/local/ptr_private.wgsl.expected.wgsl
@@ -1,6 +1,6 @@
 var<private> i : i32 = 123;
 
-[[stage(compute)]]
+[[stage(compute), workgroup_size(1)]]
 fn main() {
   let p : ptr<private, i32> = &(i);
   let use : i32 = (*(p) + 1);