Test non-crashing updating of worker scope event handlers.

LayoutTests/fast/workers/resources/shared-worker-replace-event-handler.js

+onconnect = function(event) {
+    function update() {
+        onerror = undefined;
+    }
+    for (var i = 0; i < 8; ++i) {
+        update();
+    }
+    event.ports[0].postMessage("'onerror' repeatedly updated ok.");
+    event.ports[0].postMessage("DONE:");
+};