[Extensions] Remove extension reloading in EventsToStoppedWorker test.

Loading unpacked extension causes an extension to reload quickly, which
makes EventsToStoppedWorker test heavily flaky with SW storage migration CL
(ref: https://chromium-review.googlesource.com/c/chromium/src/+/2243071).
This CL packs and loads the extension instead to avoid the reload.

Bug: 1095959
Test: Locally running the test 2000 times didn't flake in cloudtop linux build.
Change-Id: I79f37a4e5721d1d995fddb20f31c6efb95066e60
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2318508Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791416}

chrome/browser/extensions/service_worker_apitest.cc

@@ -1660,9 +1660,11 @@ IN_PROC_BROWSER_TEST_F(ServiceWorkerBasedBackgroundTest,
 
   ExtensionTestMessageListener event_listener_added("ready", false);
   event_listener_added.set_failure_message("ERROR");
+
+  // Note: Extension is packed to avoid reloading while loading.
   const Extension* extension = LoadExtensionWithFlags(
-      test_data_dir_.AppendASCII(
-          "service_worker/worker_based_background/events_to_stopped_worker"),
+      PackExtension(test_data_dir_.AppendASCII(
+          "service_worker/worker_based_background/events_to_stopped_worker")),
       kFlagNone);
   ASSERT_TRUE(extension);