Offer User Timing in workers

LayoutTests/fast/workers/resources/performance.js

-
 function log(message)
 {
     document.getElementById("result").innerHTML += message + "<br>";
 }
 
 if (window.testRunner) {
     testRunner.dumpAsText();
     testRunner.waitUntilDone();
 }
 
 var worker = createWorker();
-worker.postMessage("eval self.MessageEvent = 'PASS'; MessageEvent;");
-worker.postMessage("eval foo//bar");
+
+worker.postMessage("ping");
+worker.postMessage("eval importScripts('worker-performance.js')");
 
 worker.onmessage = function(evt) {
-    if (!/foo\/\/bar/.test(evt.data))
-        log(evt.data);
-    else {
-        log("DONE");
+    log(evt.data);
+    if (evt.data == "DONE") {
         if (window.testRunner)
             testRunner.notifyDone();
     }
 }