[XMLHttpRequest] Stop throwing for network error in async mode

LayoutTests/fast/files/workers/resources/worker-apply-blob-url-to-xhr.js

-function log(message)
-{
-    postMessage(message);
-}
-
-function sendXMLHttpRequest(method, url)
-{
-    var xhr = new XMLHttpRequest();
-    xhr.open(method, url, false);
-    try {
-        xhr.send();
-        log("Status: " + xhr.status);
-        log("Response: " + xhr.responseText);
-    } catch (error) {
-        log("Received exception, code: " + error.code + ", name: " + error.name + ", message: " + error.message);
-    }
-}
-
-onmessage = function(event)
-{
-    var file = event.data;
-    var fileURL = URL.createObjectURL(file);
-
-    log("Test that XMLHttpRequest GET succeeds.");
-    sendXMLHttpRequest("GET", fileURL);
-
-    log("Test that XMLHttpRequest POST fails.");
-    xhr = new XMLHttpRequest();
-    sendXMLHttpRequest("POST", fileURL);
-
-    log("Test that XMLHttpRequest GET fails after the blob URL is revoked.");
-    URL.revokeObjectURL(fileURL);
-    sendXMLHttpRequest("GET", fileURL);
-
-    log("DONE");
-}