[dom] Use `commonAncestorContainer` in concept-range-clone

Ms2ger has just created a new issue for https://github.com/whatwg/dom:

== Use `commonAncestorContainer` in concept-range-clone ==
https://dom.spec.whatwg.org/#concept-range-clone

> * 4. Let common ancestor be original start node.
> * 5. While common ancestor is not an inclusive ancestor of original 
end node, set common ancestor to its own parent. 

This is *range*'s `commonAncestorContainer`.

See https://github.com/whatwg/dom/issues/56