title: XMLHttpRequest Response empty in chrome Extension
body: <p>Hello I develop a chrome extension.
  In my js script i want to make an XMLHttpRequest.
  The php file is read by the script (i see it in the log of my wamp) but the xhr.status value is 0 and  the responseText is empty.</p>
  
  <pre><code>var xhr = new XMLHttpRequest();
  xhr.open("GET", 'http://localhost/ss/test.php?tt='+tt, true);
  xhr.responseType = 'text';
  xhr.onreadystatechange = function() {
      if (xhr.readyState == XMLHttpRequest.DONE) {            
          if(xhr.responseText=='a'){
              alert('a');
          }else if(xhr.responseText=='b'){
              alert('b');
          }else{
              alert(xhr.status);
          }
      }else{
          alert(xhr.responseText);
      }
  }
  xhr.send();
  </code></pre>
  
  <p>in the php file I just wrote echo "a";</p>