Re: [whatwg/dom] Interface NodeFilter - return values need a new option. (#1005)

I'm detecting focusable descendants within a whole set of focusable "containers" for A11Y purposes.

I have a single `TreeWalker` who's root is the outermost element which contains these visitable containers.

On `focusIn`, I collect the focusable descendants:

```
initialize() {
    const me = this;

    // Listen for focusin events so we can catch focus landing on our cells.
    me.element.addEventListener('focusin', me.onFocusIn.bind(me));

    me.treeWalker = document.createTreeWalker(me.element, nodeFilter.SHOW_ELEMENT, {
        acceptNode(node) {
            // Walked outside our current cell. Stop traverse.
            // We don't want it to walk on to the end of the document!
            if (!me.currentCell.contains(node)) {
                return NodeFilter.FILTER_ABORT;
            }
            // Found a focusable node, visit it.
            if (MyHelperClass.isFocusable(node)) {
                return NodeFilter.FILTER_ACCEPT;
            }
            // Try next node
            return NodeFilter.FILTER_SKIP;
        }
    });
}

onFocusIn(focusInEvent) {
    // Stepped on a container cell...
    if (focusInEvent.target.matches(this.visitableCellSelector)) {
        this.focusableDescendants = [];

        // Capture our "currentCell", so that our node accepter can tell when we've traversed outside of it.
        this.treeWalker.currentNode = this.currentCell = focusInEvent.target;

        // Collect all our focusable descendants
        for (let focusable = this.treeWalker.nextNode(); focusable; focusable =  this.treeWalker.nextNode()) {
            this.focusableDescendants.push(focusable);
        }
}
```

Warning: Code is typed in. May contain syntax errors!

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