[shadow-dom] Counters and list item counting

Currently in Webkit list item counting is done on the render tree, but we
are looking at making it use the DOM instead so that ordered lists work
properly in regions. This raises an interesting question about if they
should use the composed shadow tree, or the original tree.

ex.

<x-widget>
<ol>
  <li>
  <li>
</ol>
</x-widget>

inside x-widget:

<div>
  <content select="li:last-child">
</div>

What's the count on that projected list item?

This also raises questions of how counters interact with shadows. Should
counters work on the project DOM or the original DOM?

We're leaning towards the original DOM since otherwise counters become
difficult to work with when they're reprojected deeper and deeper down a
component hierarchy.

- E