* Output of `node_modules/.bin/electron --version`: v2.0.5
* Operating System (Platform and Version): Ubuntu 18.04LTS

### Expected behavior

alert box should show the app Icon.

### Actual behavior

alert box is showing no icon even though an app icon is set.
![screenshot from 2018-10-22 18-46-46](https://user-images.githubusercontent.com/15051871/47294523-f5773180-d62a-11e8-9a47-6a62bebd4a1d.png)


### How to reproduce
* Open any electron app in debug mode (or run the app directly from the source).
* in the developer console, type `alert('some_string')`
* A dialog box appears with 'some_string' in it and a placeholder for the icon instead of the icon itself.
Refer the image above.

atom/browser/ui/message_box_gtk.cc
const std::string lower = base::ToLowerASCII(text);
    if (lower == "cancel")
      return _("_Cancel");
    if (lower == "no")
      return _("_No");
    if (lower == "ok")
      return _("_OK");
    if (lower == "yes")
      return _("_Yes");
    return text.c_str();
  }
  void Show() {
    gtk_widget_show_all(dialog_);
    
    
    int time = ui::X11EventSource::GetInstance()->GetTimestamp();
    gtk_window_present_with_time(GTK_WINDOW(dialog_), time);
  }
  int RunSynchronous() {
    Show();
    int response = gtk_dialog_run(GTK_DIALOG(dialog_));
    if (response < 0)
      return cancel_id_;
    else
      return response;
  }
  void RunAsynchronous(const MessageBoxCallback& callback) {