[whatwg/xhr] Add link to declarative shadow dom opt-in (#300)

This PR goes along with the two in [HTML](https://github.com/whatwg/html/pull/5465) and [DOM](https://github.com/whatwg/dom/pull/892) to add declarative Shadow DOM to the spec. This PR goes along with the discussion at [#912](https://github.com/whatwg/dom/issues/912), to opt-in to the fragment parser for declarative Shadow DOM.

- [ ] At least two implementers are interested (and none opposed):
   * This is implemented, behind a flag, in Chromium. Tracking bug is [here](https://crbug.com/1042130).
   * 
- [X] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * In WPT: https://wpt.fyi/results/shadow-dom/declarative?label=master&label=experimental&aligned&q=declarative
- [X] [Implementation bugs](https://github.com/whatwg/meta/blob/master/MAINTAINERS.md#handling-pull-requests) are filed:
  *  Chrome: https://crbug.com/1042130
  * Firefox: None yet
  * Safari: None yet


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<a href="https://whatpr.org/xhr/300.html" title="Last updated on Nov 24, 2020, 6:45 PM UTC (0344579)">Preview</a> | <a href="https://whatpr.org/xhr/300/17980b7...0344579.html" title="Last updated on Nov 24, 2020, 6:45 PM UTC (0344579)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/xhr/pull/300

-- Commit Summary --

  * Set include shadow roots to "deny"
  * Add name, convert to magic anchor

-- File Changes --

    M xhr.bs (8)

-- Patch Links --

https://github.com/whatwg/xhr/pull/300.patch
https://github.com/whatwg/xhr/pull/300.diff

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