Iframe's window size is not available synchronously in about:blank

```html
<iframe></iframe>
<script>console.log(document.querySelector('iframe').contentWindow.innerWidth)</script>
```
This prints 0, because the `NewLayoutInfo` in `HTMLIFrameElement::navigate_or_reload_child_browsing_context` provides a `None` value for `window_size`, which is what is stored in the new `Window` that is created and retrieved by `Window::GetInnerWidth`.