[iOS] Ensure persistent cookie store's backend is closed on shutdown

This CL uses TaskShutdownBehavior::BLOCK_SHUTDOWN for the
background task runner associated with an
SQLLitePersistentCookieStore, matching the behavior in
content/. This ensures that when an SQLLitePersistentCookieStore
is destroyed, the call to
SQLitePersistentStoreBackendBase::DoCloseInBackground posted to
the background thread gets executed before the
SQLitePersistentStoreBackendBase is destroyed, fixing an
assertion failure.

Bug: 1121986
Change-Id: I77c7fad335b07508fb316fc6b1a88b822692893d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398510Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Ali Juma <ajuma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805287}

ios/chrome/browser/net/cookie_util.mm

@@ -48,7 +48,8 @@ scoped_refptr<net::SQLitePersistentCookieStore> CreatePersistentCookieStore(
       new net::SQLitePersistentCookieStore(
           path, base::CreateSingleThreadTaskRunner({web::WebThread::IO}),
           base::ThreadPool::CreateSequencedTaskRunner(
-              {base::MayBlock(), base::TaskPriority::BEST_EFFORT}),
+              {base::MayBlock(), base::TaskPriority::BEST_EFFORT,
+               base::TaskShutdownBehavior::BLOCK_SHUTDOWN}),
           restore_old_session_cookies, crypto_delegate));
 }