register(node, inertRoot) {
    let inertNode = this._managedNodes.get(node);
    if (inertNode !== undefined) {  // node was already in an inert subtree
      inertNode.addInertRoot(inertRoot);
      // Update saved tabindex value if necessary
      inertNode.ensureUntabbable();
    } else {
      inertNode = new InertNode(node, inertRoot);
    }

    this._managedNodes.set(node, inertNode);

    return inertNode;
  }
Register the given InertRoot as managing the given node.
In the case where the node has a previously existing inert root, this inert root will
be added to its set of inert roots.
@param {Node} node
@param {InertRoot} inertRoot
@return {InertNode} inertNode