Metal: Run vertex pulling when using Tint

The config was already being set, but the transform itself was not
being added to the manager.

Bug: dawn:571
Change-Id: I850d5b0f37635598427b4a3a3285d243b89fad46
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/54721
Commit-Queue: James Price <jrprice@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/metal/ShaderModuleMTL.mm

@@ -105,6 +105,7 @@ namespace dawn_native { namespace metal {
 
         if (stage == SingleShaderStage::Vertex &&
             GetDevice()->IsToggleEnabled(Toggle::MetalEnableVertexPulling)) {
+            transformManager.Add<tint::transform::VertexPulling>();
             AddVertexPullingTransformConfig(*vertexState, entryPointName, kPullingBufferBindingSet,
                                             &transformInputs);