Window onerror is not triggered bug fix

As pointed out in the bug, unhandled errors on worker threads were not
properly being propagated for the parent thread to handle as the spec
requires (https://html.spec.whatwg.org/multipage/workers.html#runtime-script-errors-2).
This fix adds in the code to do that.

Bug: 685303
Change-Id: Ie3e7ec03153a43efdc6e70b874d6b1a1d8ac35ef

workers/modules/dedicated-worker-import-failure.html

@@ -2,6 +2,7 @@
 <title>DedicatedWorker: import failure</title>
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
+<script> setup({allow_uncaught_exception: true}); </script>
 <script>
 
 promise_test(async () => {

workers/modules/resources/new-worker-window.html

@@ -9,7 +9,10 @@
 window.onmessage = e => {
   worker = new Worker(e.data, { type: 'module' });
   worker.onmessage = msg => window.opener.postMessage(msg.data, '*');
-  worker.onerror = err => window.opener.postMessage(['ERROR'], '*');
+  worker.onerror = err => {
+      window.opener.postMessage(['ERROR'], '*');
+      err.preventDefault();
+  };
 };
 window.opener.postMessage('LOADED', '*');
 </script>