* 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
if (Browser::Get()->is_ready()) {
    GtkMessageBox(nullptr, MESSAGE_BOX_TYPE_ERROR, {"OK"}, -1, 0, "Error",
                  base::UTF16ToUTF8(title).c_str(),
                  base::UTF16ToUTF8(content).c_str(), "", false)
        .RunSynchronous();
  } else {
    fprintf(stderr,
            ANSI_TEXT_BOLD ANSI_BACKGROUND_GRAY ANSI_FOREGROUND_RED
            "%s\n" ANSI_FOREGROUND_BLACK "%s" ANSI_RESET "\n",
            base::UTF16ToUTF8(title).c_str(),
            base::UTF16ToUTF8(content).c_str());
  }
}
}