* 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
gtk_widget_destroy(dialog_);
    if (parent_) {
      parent_->RemoveObserver(this);
      static_cast<NativeWindowViews*>(parent_)->SetEnabled(true);
    }
  }
  GtkMessageType GetMessageType(MessageBoxType type) {
    switch (type) {
      case MESSAGE_BOX_TYPE_INFORMATION:
        return GTK_MESSAGE_INFO;
      case MESSAGE_BOX_TYPE_WARNING:
        return GTK_MESSAGE_WARNING;
      case MESSAGE_BOX_TYPE_QUESTION:
        return GTK_MESSAGE_QUESTION;
      case MESSAGE_BOX_TYPE_ERROR:
        return GTK_MESSAGE_ERROR;
      default:
        return GTK_MESSAGE_OTHER;
    }
  }
  const char* TranslateToStock(int id, const std::string& text) {