Types mismatch in Document.prototype.documentElement externs

The following JS input
```js
new DOMParser()
.parseFromString(``, "image/svg+xml")
.documentElement
.width
.baseVal
```

compiles into this under advanced optimizations
```js
new DOMParser()
.parseFromString(``, "image/svg+xml")
.documentElement
.width
.g
```

The `baseVal` property gets minified and the following warning is printed `WARNING - [JSC_INEXISTENT_PROPERTY] Property width never defined on HTMLHtmlElement`.

As per externs, `DOMParser.prototype.parseFromString` return type is `Document`
https://github.com/google/closure-compiler/blob/7ff6e25843097791416d1544c88dc8711f6be64a/externs/browser/gecko_xml.js#L73, whose `documentElement` property type is indeed `HTMLHtmlElement` https://github.com/google/closure-compiler/blob/7ff6e25843097791416d1544c88dc8711f6be64a/externs/browser/w3c_dom1.js#L339

But in case when parsing SVG the `documentElement` property refers to an element of type `SVGSVGElement`, which is correctly covered with externs https://github.com/google/closure-compiler/blob/7ff6e25843097791416d1544c88dc8711f6be64a/contrib/externs/svg.js#L6555