[Offline Prefetch] Backoff support for PrefetchBackgroundTask

components/offline_pages/content/suggested_articles_observer_unittest.cc

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/offline_pages/content/suggested_articles_observer.h"
 
 #include "base/run_loop.h"
 #include "base/test/test_simple_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "components/offline_pages/core/client_namespace_constants.h"
@@ skipping 35 matching lines @@
   }
 
   void RemovePrefetchURLsByClientId(const ClientId& client_id) override {
     DCHECK_EQ(client_id.name_space, kSuggestedArticlesNamespace);
     remove_by_client_id_count++;
     last_removed_client_id = base::MakeUnique<ClientId>(client_id);
   }
 
   void BeginBackgroundTask(
       std::unique_ptr<ScopedBackgroundTask> task) override {}
-  void StopBackgroundTask(ScopedBackgroundTask* task) override {}
+  void StopBackgroundTask() override {}
+  void RequestFinishBackgroundTaskForTest() {}
 
   std::vector<PrefetchURL> latest_prefetch_urls;
   std::unique_ptr<ClientId> last_removed_client_id;
 
   int new_suggestions_count = 0;
   int remove_all_suggestions_count = 0;
   int remove_by_client_id_count = 0;
 };
 
 class TestingPrefetchService : public PrefetchService {
@@ skipping 139 matching lines @@
 
   observer()->OnSuggestionInvalidated(
       ntp_snippets::ContentSuggestion::ID(category, test_url_1.spec()));
 
   EXPECT_EQ(1, test_prefetch_dispatcher()->remove_by_client_id_count);
   EXPECT_EQ(ClientId(kSuggestedArticlesNamespace, test_url_1.spec()),
             *test_prefetch_dispatcher()->last_removed_client_id);
 }
 
 }  // namespace offline_pages