Catch uncaught promise rejections from V8 and log to console.

LayoutTests/http/tests/serviceworker/resources/test-helpers.js

 // Adapter for testharness.js-style tests with Service Workers
 
 function service_worker_unregister_and_register(test, url, scope) {
   if (!scope || scope.length == 0)
     return Promise.reject(new Error('tests must define a scope'));
 
   var options = { scope: scope };
   return service_worker_unregister(test, scope)
       .then(function() {
           return navigator.serviceWorker.register(url, options);
@@ skipping 237 matching lines @@
 }
 
 function test_login(test, origin, username, password) {
   return new Promise(function(resolve, reject) {
       with_iframe(
         origin + base_path() +
         'resources/fetch-access-control-login.html')
         .then(test.step_func(function(frame) {
             var channel = new MessageChannel();
             channel.port1.onmessage = test.step_func(function() {
-                unload_iframe(frame);
+                unload_iframe(frame).catch(function() {});
                 resolve();
               });
             frame.contentWindow.postMessage(
               {username: username, password: password},
               [channel.port2], origin);
           }));
     });
 }