overrideMimeType should be called after open to be spec compliant

chrome/test/data/extensions/subscribe_page_action/subscribe.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Grab the querystring, removing question mark at the front and splitting on
 // the ampersand.
 var queryString = location.search.substring(1).split("&");
 
 // The feed URL is the first component and always present.
 var feedUrl = decodeURIComponent(queryString[0]);
@@ skipping 109 matching lines @@
   } else {
     // Normal loading just requires links to the css and the js file.
     styleSheet = "<link rel='stylesheet' type='text/css' href='" +
                     chrome.extension.getURL("style.css") + "'>";
     frameScript = "<script src='" + chrome.extension.getURL("iframe.js") +
                   "'></" + "script>";
   }
 
   req.onload = handleResponse;
   req.onerror = handleError;
+  req.open("GET", feedUrl, !synchronousRequest);
   // Not everyone sets the mime type correctly, which causes handleResponse
   // to fail to XML parse the response text from the server. By forcing
   // it to text/xml we avoid this.
   req.overrideMimeType('text/xml');
-  req.open("GET", feedUrl, !synchronousRequest);
   req.send(null);
 
   document.getElementById('feedUrl').href = 'view-source:' + feedUrl;
 }
 
 // Sets the title for the feed.
 function setFeedTitle(title) {
   var titleTag = document.getElementById('title');
   titleTag.textContent =
       chrome.i18n.getMessage("rss_subscription_feed_for", title);
@@ skipping 111 matching lines @@
   var button = document.getElementById('rss_subscription_subscribe_button');
   button.addEventListener('click', navigate);
 
   main();
 });
 
 window.addEventListener("message", function(e) {
   if (e.ports[0] && e.data === token)
     e.ports[0].postMessage(req.responseText);
 }, false);