Re: [webcomponents] Remove the constraint that slot can't have another slot as an ancestor (#321)

It doesn't.  Allowing nested slots would not cause a single event to be dispatched on siblings of a single parent node.

For example, let us examine the following very simple shadow tree:
```html
<slot id=s1 name=foo>
  <span id=s2>
    <slot id=s3 name=bar></slot>
  </span>
</slot>
```

If an event were to fire at a node assigned to `s1`, then it'll just propagate up to the shadow root in the shadow tree, and goes to the shadow host.  Nothing special about that.  If an event fires at a node assigned to `s3`, it would similarly propagate to `s2`, `s1`, and then to `s1`'s parent.  None of the nodes assigned to `s1` comes into play in that scenario.

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