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

After removing, I found that an issue of removing the constraint:

```html
<slot id=s1 name=foo>
  <span id=s2>
    <slot id=s3 name=bar></slot>
  </span>
</slot>
```

> 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.

Assuming that both s1 and s3 have their assigned node, 
If an event fires at a node assigned to s3, call it *bar*, the event path would be:

- The event path: [bar, s3, s2, s1, host, ...]

However, this wouldn't match the ancestor chain of the composed tree. :(

e.g. see `s2` and `s1` (or `s2` and `host` if we omit slots from the event path). There is no relationship between them in the composed tree.


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