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

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

-if (window.testRunner) {
-    testRunner.dumpAsText();
-    testRunner.waitUntilDone();
-}
-
-var console_messages = document.createElement("ul");
-document.body.appendChild(console_messages);
-
-function log(message)
-{
-    var item = document.createElement("li");
-    item.appendChild(document.createTextNode(message));
-    console_messages.appendChild(item);
-}
-
-var progress_ticks = 0;
-var worker = createWorker('resources/xmlhttprequest-timeout-override-worker.js');
-var messages = [];
-worker.onmessage = function(evt)
-{
-    if (/log .+/.test(evt.data)) {
-        var msg = evt.data.substr(4);
-        messages.push(msg);
-        log(msg);
-    } else if (/DONE/.test(evt.data)) {
-        log(messages.length === 1 && messages[0] === "timeout" ? "PASS" : "FAIL");
-        if (window.testRunner)
-            testRunner.notifyDone();
-    }
-}