Using XMLHttpRequests in Javascript

Chrome extensions are allo开发者_高级运维wed to make XMLhttpRequests outside of your own domain if you specify them in your Json file. However, after I have done this, I still get status 0 when trying to make an XMLhttpRequest. Does anyone know how to fix this?

My javascript:

var xhr = new XMLHttpRequest();
xhr.onreadystatechange = function() {
   if (xhr.readyState == 4) {
   alert(xhr.status);
       if (xhr.status == 200) {
       alert(xhr.responseText);
       }
   }   
}
var url = "http://search.twitter.com/trends/current.json?exclude=hashtags";
xhr.open("GET", url, true);
xhr.send();

My Json file

{
  "name": "My First Extension",
  "version": "1.0",
  "description": "The first extension that I made.",
  "browser_action": {
    "default_icon": "icon.png",
    "popup": "popup.html"
  },
  "permissions": [
    "http://search.twitter.com/"
  ]
}

This is the link to the google example http://src.chromium.org/viewvc/chrome/trunk/src/chrome/common/extensions/docs/examples/howto/contentscript_xhr/background.html?view=markup

Looks like that permissions are related to URLs not to hosts.
Try using a wildcard

http://search.twitter.com/*

http://code.google.com/chrome/extensions/match_patterns.html