* 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_hide(dialog_);
  if (response < 0)
    callback_.Run(cancel_id_, checkbox_checked_);
  else
    callback_.Run(response, checkbox_checked_);
  delete this;
}
void GtkMessageBox::OnCheckboxToggled(GtkWidget* widget) {
  checkbox_checked_ = gtk_toggle_button_get_active(GTK_TOGGLE_BUTTON(widget));
}
}  
int ShowMessageBox(NativeWindow* parent,
                   MessageBoxType type,
                   const std::vector<std::string>& buttons,
                   int default_id,
                   int cancel_id,
                   int options,
                   const std::string& title,
                   const std::string& message,
                   const std::string& detail,
                   const gfx::ImageSkia& ) {
  return GtkMessageBox(parent, type, buttons, default_id, cancel_id, title,
                       message, detail, "", false)
      .RunSynchronous();
}
void ShowMessageBox(NativeWindow* parent,