resolveRelativeUrl = function(relativeURL) {
  return resolveUrl(relativeURL, urlBase);
}
Resolves the given relative URL string without requiring a specific base URL
(unlike resolveUrl). Will resolve the relative URL against the current
document's BaseURI, and the resulting URL WILL contain properties from
this URI.
@param {string} relativeURL A string which may be only a relative URL (i.e.
    has no protocol, userinfo, hostname, or port).
@return {!UrlLike} A URL that is relative to the current document's Base URI
    with all the relevant relative parts from the input parameter.