Allow XHR timeout attribute to be overridden after send(), per spec

LayoutTests/http/tests/xmlhttprequest/workers/resources/xmlhttprequest-timeout-override-worker.js

+importScripts("worker-pre.js");
+
+function log(message) {
+    postMessage("log " + message);
+}
+
+function done() {
+    postMessage("DONE");
+}
+
+function eventHandler(e) {
+    log(e.type);
+    done();
+}
+
+function init() {
+    try {
+        var xhr = new XMLHttpRequest();
+        xhr.ontimeout = eventHandler;
+        xhr.onabort = eventHandler;
+        xhr.onerror = eventHandler;
+        xhr.onload = eventHandler;
+
+        xhr.timeout = 100000;
+        xhr.open("GET", "../../../resources/load-and-stall.php?name=../resources/test.mp4&stallAt=0&stallFor=1000&mimeType=video/mp4", true);
+
+        // Defer overriding timeout
+        setTimeout(function() {
+            xhr.timeout = 400;
+        }, 200);
+
+        setTimeout(function() {
+            xhr.abort();
+        }, 1000);
+
+        xhr.send();
+    } catch (e) {
+        log(e);
+        done();
+    }
+};