getHamburgerMenu() {
    const menu = document.getElementById('hamburger-menu-contents');
    return /** @type {!HTMLDivElement} */ (menu);
  }
Gets the hamburger menu's content div, so that the caller to add elements
to it.
There is no guarantee that the caller is the only entity that has added
contents to the hamburger menu.
@return {!HTMLDivElement} The container for the hamburger menu.