[webcomponents] getDistributedNodes() shouldn't exclude a node in closed shadow tree. (#109)

The multiple shadow roots are gone, so getDistributedNodes() doesn't need to exclude a node in closed shadow trees.
The distributed nodes are always in ancestor trees of the node tree which insertion points participates in.
That means the distributed nodes are always unclosed node of insertion points, in theory.

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/109