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

This seems like a completely unwarranted constraint:
> There is no other slot element in the ancestors of the slot element

This requirement introduces an inconsistency in the way slots are found in the shadow tree. e.g. `s3` instead of `s2` is the active slot in the example below despite of the fact `s2` appears before `s3` in the tree order:
```html
<slot id=s1 name=foo>
  <slot id=s2 name=bar></slot>
</slot>
<slot id=s3 name=bar></slot>
```

It also prevents a rather compelling use case of nested slots once the fallback contents are supported by slot elements (see the issue #317).  e.g.
```html
<slot name=fullName>
  <slot name=firstName></slot> <slot name=lastName></slot>
</slot>
```


---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/321