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/intrinsics/gen/modf/546e09.wgsl.expected.wgsl b/test/intrinsics/gen/modf/546e09.wgsl.expected.wgsl
index 214b34d..331c087 100644
--- a/test/intrinsics/gen/modf/546e09.wgsl.expected.wgsl
+++ b/test/intrinsics/gen/modf/546e09.wgsl.expected.wgsl
@@ -14,7 +14,7 @@
   modf_546e09();
 }
 
-[[stage(compute)]]
+[[stage(compute), workgroup_size(1)]]
 fn compute_main() {
   modf_546e09();
 }