Set MessageEvent.source to the newly created port for shared workers' connect events

LayoutTests/fast/workers/resources/messageevent-source.js

+onconnect = function(event) {
+    if (event.__proto__ === MessageEvent.prototype)
+        event.ports[0].postMessage("PASS: event.__proto__ is MessageEvent.prototype");
+    else
+        event.ports[0].postMessage("FAIL: event.__proto__ is not MessageEvent.prototype");
+    if (event.source === event.ports[0])
+        event.ports[0].postMessage("PASS: event.source is event.ports[0]");
+    else
+        event.ports[0].postMessage("FAIL: event.source is not event.ports[0]");
+    event.ports[0].postMessage("DONE:");
+};