It's going to be removed from Chromium, see https://crbug.com/812523.

/cc @electron/code-owners 
shell/browser/api/electron_api_app.cc
v8::Local<v8::Value> val;
  args->GetNext(&val);
  if (val->IsNull()) {
    delegate->ContinueWithCertificate(nullptr, nullptr);
    return;
  }
  gin_helper::Dictionary cert_data;
  if (!gin::ConvertFromV8(isolate, val, &cert_data)) {
    args->ThrowError("Must pass valid certificate object.");
    return;
  }
  std::string data;
  if (!cert_data.Get("data", &data))
    return;
  auto certs = net::X509Certificate::CreateCertificateListFromBytes(
      data.c_str(), data.length(), net::X509Certificate::FORMAT_AUTO);
  if (!certs.empty()) {
    scoped_refptr<net::X509Certificate> cert(certs[0].get());
    for (size_t i = 0; i < identities->size(); ++i) {
      if (cert->EqualsExcludingChain((*identities)[i]->certificate())) {
        net::ClientCertIdentity::SelfOwningAcquirePrivateKey(
            std::move((*identities)[i]),
            base::BindRepeating(&GotPrivateKey, delegate, std::move(cert)));
        break;
      }
    }
  }
}
#if defined(USE_NSS_CERTS)
int ImportIntoCertStore(CertificateManagerModel* model,