[whatwg/dom] Add is-available-to-element-internals (#893)

Add `"is available to element internals"` flag, for use with the accompanying [HTML PR](https://github.com/whatwg/html/pull/5921) which restricts access to `ElementInternals.shadowRoot` for shadow roots which were pre-existing before a custom element upgrade/construction. See the discussion on [Issue 871](https://github.com/w3c/webcomponents/issues/871#issuecomment-692383222) for more context.

- [X] At least two implementers are interested (and none opposed):
   * Chromium - me.
   * WebKit: see [this comment](https://github.com/w3c/webcomponents/issues/871#issuecomment-692383222).
- [X] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
   * Will add link soon - WPT has been written.
- [X] [Implementation bugs](https://github.com/whatwg/meta/blob/master/MAINTAINERS.md#handling-pull-requests) are filed:
   * Chrome: https://crbug.com/1042130

   * Firefox: …
   * Safari: …

You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/dom/pull/893


-- Commit Summary --

  * Add is-available-to-element-internals

-- File Changes --

    M dom.bs (8)

-- Patch Links --

https://github.com/whatwg/dom/pull/893.patch

https://github.com/whatwg/dom/pull/893.diff


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