_adoptInertRoot(node) {
    let inertSubroot = this._inertManager.getInertRoot(node);

    // During initialisation this inert root may not have been registered yet,
    // so register it now if need be.
    if (!inertSubroot) {
      this._inertManager.setInert(node, true);
      inertSubroot = this._inertManager.getInertRoot(node);
    }

    for (let savedInertNode of inertSubroot.managedNodes)
      this._manageNode(savedInertNode.node);
  }
If a descendant node is found with an `inert` attribute, adopt its managed nodes.
@param {Node} node