LibWeb: Wait until new document is active before running SVG scripts

This is the same fix as 07928129dd62fc8478fd1e9967efe4cd16909340
also applied to the SVG script element case. Fixes a crash for the
following HTML:

```html
<svg>
<script>
location.reload();
</script>
</svg>
```

As with the linked commit:
> With this change WebContent does not crash when `location.reload()` is
> invoked but `Navigable::reload()` still not working because of spec
> issue (whatwg/html#9869) so we can't add a
> test yet.