message: Enable Metal validation layers
diff: #include "dawn_native/metal/BackendMTL.h"
  +#include "dawn_native/Instance.h"
  #include "dawn_native/MetalBackend.h"
  #include "dawn_native/metal/DeviceMTL.h"
  @@ -152,6 +153,9 @@ namespace dawn_native { namespace metal {
  // Implementation of the Metal backend's BackendConnection
  Backend::Backend(InstanceBase* instance) : BackendConnection(instance, BackendType::Metal) {
  + if (GetInstance()->IsBackendValidationEnabled()) {
  + setenv("METAL_DEVICE_WRAPPER_TYPE", "1", 1);
  + }
  }
  std::vector<std::unique_ptr<AdapterBase>> Backend::DiscoverDefaultAdapters() {
  