Bug 244972

IDREF element reflection hasn't been hooked up with AccessibilityObject.

Right now if you do something like the following example, the a11y tree doesn't know anything about the label you're assigning to the custom input:
        <label id="anotherLabel">Another label</label>
        <x-input-idref id="myxinput"></x-input-idref>

        <script>
          class XInputIDREF extends HTMLElement {
            constructor() {
              super();
              this.attachShadow({ mode: "open" });
              let input = document.createElement("input");
              input.id = "innerbutton";
              this.shadowRoot.appendChild(input);
              input.ariaLabelledByElements = [anotherLabel];
            }
          }
          customElements.define("x-input-idref", XInputIDREF);

If you use the accessibility info in the inspector, you'll see the inner <input> doesn't have the label associated to it.
This is because as the attribute is not reflected, as the label is not in the shadow tree, and we need to update AccessibilityObject::elementsFromAttribute() to return the explicitly set elements in that scenario.