Re: [dom] DOMTokenList hook to validate added token is in a supported set of keywords (#103)

The feature detection story is basically:
```
    var link = document.createElement("link");
    if (link.relList && link.relList.add("preload")) {
        // supported, since add() returned true
    } else {
        // Not supported. One of three options:
        // * relList not implemented
        // * add() returned "undefined", since this spec change wasn't yet implemented
        // * add() returned "false" since "preload" is not a valid token
    }
```

The above example refers to `<link>`, but the story is similar for `<iframe>`.

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