Convert CookieMonsterTest#FlushStore to use OnceCallback

Bug: 1007815
Change-Id: I225112c7fcd440a7f67980ccca934593d6110e1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2083182Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Anna Malova <amalova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746376}

net/cookies/cookie_monster_unittest.cc

@@ -2411,7 +2411,7 @@ TEST_F(CookieMonsterTest, FlushStore) {
   ASSERT_EQ(1, counter->callback_count());
 
   // NULL callback is safe.
-  cm->FlushStore(base::Closure());
+  cm->FlushStore(base::OnceClosure());
   base::RunLoop().RunUntilIdle();
 
   ASSERT_EQ(0, store->flush_count());