[whatwg/dom] No way to resolve a Promise with a cross domain WindowProxy (#536)

Since resolving a promise looks for a thenable and accessing the `then` property on a cross domain WindowProxy throws you can't do something like:

```js
const loadFrame = (src) => new Promise((resolve) => {
  frame.src = src;
  frame.onload = () => resolve(frame.contentWindow);
});
```

Perhaps cross domain WindowProxy objects should allow accessing `then` and have it always be undefined?

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