initTenantList_(onClick) {
    const buttons =
        this.getElementsByClass('firebaseui-id-tenant-selection-button');
    const cb = (tenantId, e) => {
      onClick(tenantId);
    };
    for (let i = 0; i < buttons.length; i++) {
      const button = buttons[i];
      const tenantId = dataset.get(button, 'tenantId');
      element.listenForActionEvent(this, button, goog.partial(cb, tenantId));
    }
  }
Initializes tenant selection menu buttons.
@param {function(?string)} onClick Callback to invoke when the user clicks
    one tenant selection button.
@private