tests: mark ServiceWorkerBasedBackgroundTest.UpdatePackedExtension flaky

TBR=lazyboy@chromium.org

Bug: 1003244
Change-Id: I88bfd3e89650a53fb6701685fa317f6cd6100f67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1906946Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713888}

chrome/browser/extensions/service_worker_apitest.cc

@@ -1190,8 +1190,9 @@ 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,
-                       UpdatePackedExtension) {
+                       DISABLED_UpdatePackedExtension) {
   constexpr char kManifest1[] =
       R"({
            "name": "Test Extension",