Name computation: Include shadow DOM in name from content

[Jym77]

See w3c/accname#167 for what accname seems to be planning.

Test page for browsers:

<div id="div1" role="button">
  <span slot="foo">Button in light, Slot in shadow</span>
</div>

<div id="div2"><span slot="foo">Button in shadow</span></div>

<script>
  const shadowRoot1 = document
    .getElementById("div1")
    .attachShadow({ mode: "open" });
  shadowRoot1.innerHTML = '<slot name="foo"></slot>';

  const shadowRoot2 = document
    .getElementById("div2")
    .attachShadow({ mode: "open" });
  shadowRoot2.innerHTML = '<button><slot name="foo"></slot></button>';
</script>

<div role="button"><iframe srcdoc="Hello"></iframe></div>