* 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
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 std::string& checkbox_label,
                    bool checkbox_checked,
                    const gfx::ImageSkia& ,
                    const MessageBoxCallback& callback) {
  (new GtkMessageBox(parent, type, buttons, default_id, cancel_id, title,
                     message, detail, checkbox_label, checkbox_checked))
      ->RunAsynchronous(callback);
}
void ShowErrorBox(const base::string16& title, const base::string16& content) {