Re: [webidl] Check if NodeFilter.length is defined correctly. (#83)

Hmm, more results for:
```
<script>
 console.log(NodeFilter.__proto__ == Function.prototype);
 console.log(typeof NodeFilter);
 console.log(NodeFilter.length);
 console.log(NodeFilter.hasOwnProperty("length"));
</script>
```
IE11: `false, object, undefined, false`
Opera 12.x: `true, function, 0, false`
Firefox and Chrome: `true, function, 0, true`

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https://github.com/heycam/webidl/issues/83#issuecomment-193693278