Fix UBO/SSBO OOB in ComputeStorageBufferBarrierTests

The test was failing on Swiftshader, likely because the allocations for
the SSBO and the UBO where neighbors. The UBO test uses uvec4 instead of
uint so it needs 4 times less invocations to perform the test.

Bug: dawn:283
Change-Id: Id1952505763a0a3eb34718e000cb386e1faa4ade
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/19082
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>

src/tests/end2end/ComputeStorageBufferBarrierTests.cpp

@@ -181,7 +181,7 @@ TEST_P(ComputeStorageBufferBarrierTests, UniformToStorageAddPingPong) {
         wgpu::ComputePassEncoder pass = encoder.BeginComputePass();
         pass.SetPipeline(pipeline);
         pass.SetBindGroup(0, bindGroups[b]);
-        pass.Dispatch(kNumValues);
+        pass.Dispatch(kNumValues / 4);
         pass.EndPass();
     }