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

/cc @electron/code-owners 
shell/browser/api/electron_api_app.cc
content::WebContents* web_contents,
    int cert_error,
    const net::SSLInfo& ssl_info,
    const GURL& request_url,
    bool is_main_frame_request,
    bool strict_enforcement,
    base::OnceCallback<void(content::CertificateRequestResultType)> callback) {
  auto adapted_callback = base::AdaptCallbackForRepeating(std::move(callback));
  v8::Locker locker(isolate());
  v8::HandleScope handle_scope(isolate());
  bool prevent_default =
      Emit("certificate-error",
           WebContents::FromOrCreate(isolate(), web_contents), request_url,
           net::ErrorToString(cert_error), ssl_info.cert, adapted_callback);
  
  if (!prevent_default)
    adapted_callback.Run(content::CERTIFICATE_REQUEST_RESULT_TYPE_DENY);
}
base::OnceClosure App::SelectClientCertificate(