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

/cc @electron/code-owners 
shell/browser/api/electron_api_app.cc
scoped_refptr<net::X509Certificate> cert,
                   scoped_refptr<net::SSLPrivateKey> private_key) {
  delegate->ContinueWithCertificate(cert, private_key);
}
void OnClientCertificateSelected(
    v8::Isolate* isolate,
    std::shared_ptr<content::ClientCertificateDelegate> delegate,
    std::shared_ptr<net::ClientCertIdentityList> identities,
    gin_helper::Arguments* args) {
  if (args->Length() == 2) {
    delegate->ContinueWithCertificate(nullptr, nullptr);
    return;
  }