Merge 120845

LayoutTests/http/tests/xmlhttprequest/reentrant-cancel.html

 <script>
 if (window.layoutTestController)
     layoutTestController.dumpAsText();
 
 function addElement() {
     document.documentElement.appendChild(document.createTextNode('X'));
 }
 document.addEventListener("DOMContentLoaded", addElement, false);
 window.onload = addElement;
 
 var xhr = new XMLHttpRequest;
 function sendXHR()
 {
     xhr.open("GET", "", true);
     xhr.send();
 }
 window.addEventListener("DOMSubtreeModified", sendXHR);
 addElement();
 </script>
 This tests that when we re-entrantly create and cancel XHRs, we don't try to disconnect the same CachedResourceClient
 multiple times from its CachedResource. We pass if we don't crash.