gistfile1.js

(function () {
  var _onload = function(){
   if (document.readyState && !/loaded|complete/.test(document.readyState)){setTimeout(_onload, 10);return}
   if (!window.__adroll_loaded){__adroll_loaded=true;setTimeout(_onload, 50);return}
   var scr = document.createElement("script");
   var host = (("https:" == document.location.protocol) ? "https://s.adroll.com" : "http://a.adroll.com");
   scr.setAttribute('async', 'true');
   scr.type = "text/javascript";
   scr.src = host + "/j/roundtrip.js";
   ((document.getElementsByTagName('head') || [null])[0] ||
    document.getElementsByTagName('script')[0].parentNode).appendChild(scr);};
  if (window.addEventListener) {window.addEventListener('load', _onload, false);}
  else {window.attachEvent('onload', _onload)}}());

You don't need line 3. For IE you can listen to script.onreadystatechange (and the subject should be the script, not the document).
Something along these lines (we use it for dinamically loading jQuery):

if (script.readyState) { // Internet Explorer
  script.onreadystatechange = function(){
    if (script.readyState === "loaded" || script.readyState === "complete") {
    script.onreadystatechange = null;
    // script loaded
    }
  };
  } 
 else { // non IE
  script.onload = function() {
    // Script loaded
  };
}

Line 3 is for WebKit, it happens to work also on IE. I don't need to know when the script is loaded, it's more when the page has finished loading so I can do my stuff from the script without having the user think that the page hasn't loaded yet.

I don't think it matters but the IIFE (like yours) are written like (function(){})(); and not (function(){}());
HTH