ServiceWorker: Introduce ExtendableMessageEvent to replace MessageEvent

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

+self.onmessage = function(e) {
+    switch (e.data) {
+    case 'WAIT':
+        e.waitUntil(new Promise(function(r) {
+            self.resolve = r;
+            self.port.postMessage('ACK');
+          }).then(function() {
+            self.port.postMessage('PASS');
+          }));
+        return;
+    case 'WAIT_ASYNC':
+        setTimeout(function() {
+            try {
+                e.waitUntil(new Promise(function() {}));
+            } catch (error) {
+                if (error.name == 'InvalidStateError')
+                    self.port.postMessage('PASS');
+                else
+                    self.port.postMessage(
+                        'FAIL: unexpected exception: ' + error);
+            }
+          }, 0);
+        return;
+    case 'DONE':
+        self.resolve();
+        return;
+    default:
+        if (e.data.port)
+            self.port = e.data.port;
+        return;
+    }
+};