[Extensions] Enable UpdatePackedExtension as the underlying problem is fixed.

https://chromium-review.googlesource.com/c/chromium/src/+/1970854 should
have fixed ServiceWorkerBasedBackgroundTest.UpdatePackedExtension test
and this CL enables the test.

Bug: 1003244
Change-Id: Iabc65e87b2f718987de007f029a63adff8992316
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1984751Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728080}

chrome/browser/extensions/service_worker_apitest.cc

@@ -1181,9 +1181,8 @@ IN_PROC_BROWSER_TEST_F(ServiceWorkerTest, MAYBE_WebAccessibleResourcesFetch) {
 // Tests that updating a packed extension with modified scripts works
 // properly -- we expect that the new script will execute, rather than the
 // previous one.
-// Flaky on all platforms: https://crbug.com/1003244
 IN_PROC_BROWSER_TEST_F(ServiceWorkerBasedBackgroundTest,
-                       DISABLED_UpdatePackedExtension) {
+                       UpdatePackedExtension) {
   constexpr char kManifest1[] =
       R"({
            "name": "Test Extension",