ServiceWorker: Wait for the completion of async tasks on ExtendableMessageEvent

third_party/WebKit/LayoutTests/http/tests/serviceworker/resources/client-id-worker.js

 self.onmessage = function(e) {
   var port = e.data.port;
   var message = [];
 
-  Promise.resolve()
+  var promise = Promise.resolve()
     .then(function() {
         // 1st matchAll()
         return self.clients.matchAll().then(function(clients) {
             clients.forEach(function(client) {
                 message.push(client.id);
               });
           });
       })
     .then(function() {
         // 2nd matchAll()
         return self.clients.matchAll().then(function(clients) {
             clients.forEach(function(client) {
                 message.push(client.id);
               });
           });
       })
     .then(function() {
         // Send an array containing ids of clients from 1st and 2nd matchAll()
         port.postMessage(message);
       });
+  e.waitUntil(promise);
 };