ServiceWorker: Introduce ExtendableMessageEvent to replace MessageEvent

LayoutTests/http/tests/serviceworker/resources/extendable-message-event-worker.js

+self.onmessage = function(e) {
+    if (e.data.port) {
+      self.port = e.data.port;
+      return;
+    }
+    if (e instanceof ExtendableMessageEvent === false) {
+        self.port.postMessage('Event type should be ExtendableMessageEvent');
+        return;
+    }
+    if (e.source instanceof Client === false &&
+        e.source.__proto__ !== registration.active.__proto__) {
+        self.port.postMessage('Source type should be Client or ServiceWorker');
+        return;
+    }
+    if (e.type !== 'message') {
+        self.port.postMessage('Event type should be "message"');
+        return;
+    }
+    var event = new ExtendableMessageEvent('eventType', {source: e.source});
+    if (event.source !== e.source) {
+        self.port.postMessage('Construct new event with source fail');
+        return;
+    }
+    self.port.postMessage('PASS');
+};