Element reflection references from inner to outer shadow roots

[chrisosaurus]

Context: #5401 (comment)

One of the core use cases for element reflection is to support an element within an inner shadow root ("darker") referencing an element held in an outer shadow root ("lighter"), as idrefs do not support this.

<div id="B"></div>
<div id="shadow-host">
# shadowRoot 
|  <div id="A"></div>
</div>

A.ariaFooElement = B; // outer tree
console.log(A.ariaFooElement);  // B

During the F2F meeting we also discussed a limited mechanism for the reverse (references from outer into inner), which has been broken out into a separate bug WICG/aom#169

The purpose of this bug is to check if we have consensus on supporting element reflection from an element within a shadow root to an element in outer dom or if there are objections to this aspect of the feature.