CookieStore: Fix Flaky WPT Test for getSubscriptions

This test failes because the cleanup to "unsubscribe" cookies would
occur after the registration has already been unregistered for
non-workers. This change removes the cleanup to "unsubscribe" and
just relised on "unregister" for cleanup. Related issue[1].

[1] https://crrev.com/c/2014100
[2] Error(Googlers): http://screen/E5X6dNVEGQz

Bug: 1047155
Change-Id: I64b62d2a15e391c3da074b2a3a5a8114b02469ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2040370Reviewed-by: Victor Costan <pwnall@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Commit-Queue: Ayu Ishii <ayui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738725}

third_party/blink/web_tests/external/wpt/cookie-store/cookieStoreManager_getSubscriptions_single.tentative.https.any.js

@@ -34,7 +34,13 @@ promise_test(async testCase => {
       { name: 'cookie-name', matchType: 'equals', url: `${scope}/path` }
     ];
     await registration.cookies.subscribe(subscriptions);
-    testCase.add_cleanup(() => registration.cookies.unsubscribe(subscriptions));
+    testCase.add_cleanup(() => {
+      // For non-ServiceWorker environments, registration.unregister() cleans up
+      // cookie subscriptions.
+      if (self.GLOBAL.isWorker()) {
+        return registration.cookies.unsubscribe(subscriptions);
+      }
+    });
   }
 
   const subscriptions = await registration.cookies.getSubscriptions();