Intent to prototype and ship: ElementInternals.shadowRoot

[Edgar Chen]

Summary: Adds the ability to retrieve the shadow root from `ElementInternals`, via the `ElementInternals.shadowRoot` attribute. This attribute is restricted to only return shadow roots created during or after the custom element constructor, for custom elements.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1723521

Standard: https://html.spec.whatwg.org/multipage/custom-elements.html#shadow-root-access

Platform coverage: all

Preference: dom.webcomponents.elementInternals.enabled

Devtools bug: N/A

Other browsers:
  * WebKit: Not implemented yet.
  * Blink: Shipped.

web-platform-tests:
https://github.com/web-platform-tests/wpt/blob/master/custom-elements/element-internals-shadowroot.html

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Edgar Chen