[whatwg/dom] Iterable NamedNodeMap (#648)

Based on Chrome, FF implementations `NamedNodeMap` can be iterable(`[Symbol.iterator]`)

![Chrome](https://user-images.githubusercontent.com/509265/40323131-2f0073ec-5d3d-11e8-9ab6-d8f2a3eba9ba.png)

![FF](https://user-images.githubusercontent.com/509265/40323110-23405446-5d3d-11e8-9ca1-954eb676f95f.png) 

```javascript
for (const attr of document.body.attributes) {
    console.log(attr);
}
```

however, in the spec, there is no mention about that. For instance [`DOMTokenList`](https://dom.spec.whatwg.org/#interface-domtokenlist) has `iterable<DOMString>`.



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