Bug 1426467: Part 1: Rewrite test_cache_worker_gc.html to use async functions. r=baku

This just cleans up the function a bit to make the next change easier to see. No
behavior change intended.

Differential Revision: https://phabricator.services.mozilla.com/D9217

--HG--
extra : moz-landing-system : lando

dom/cache/test/mochitest/test_cache_worker_gc.html

@@ -29,18 +29,19 @@ function gc() {
   });
 }
 
-var weakWorker;
-
 SimpleTest.waitForExplicitFinish();
-startWorker().then(w => {
-  weakWorker = SpecialPowers.Cu.getWeakReference(w);
+
+async function test() {
+  let w = await startWorker();
+  let weakWorker = SpecialPowers.Cu.getWeakReference(w);
   ok(weakWorker, 'worker supports weak reference');
-}).then(_ => {
-  return gc();
-}).then(_ => {
+  w = null;
+  await gc();
   ok(!weakWorker.get(), 'worker weak reference should be garbage collected');
   SimpleTest.finish();
-});
+}
+
+test();
 </script>
 </body>
 </html>