Reintroduce step 2H of the accessible name computation

[kasperisager]

Name.from() currently doesn't compute a name for cases like the following:

<h1>
  <table>
    <tr>
      <td>Hello world</td>
    </tr>
  </table>
</h1>

That is, the <h1> element isn't assigned a name even though it should according to step 2H of the accessible name computation (https://w3c.github.io/accname/#step2H). Both Chrome, Firefox, and Safari report its name as "Hello world".

The step was previously removed in #360 due to performance issues so we'll need to find of way of reintroducing this step without impacting performance. It therefore might be worthwhile looking into how this is handled in browsers.

• Follow up on DEV-10611.