Allow ObserverListThreadSafe to be used from sequenced tasks.

components/ntp_snippets/features.cc

 // Copyright 2016 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/ntp_snippets/features.h"
 
+#include "base/memory/ptr_util.h"
 #include "base/time/clock.h"
 #include "components/ntp_snippets/category_rankers/click_based_category_ranker.h"
 #include "components/ntp_snippets/category_rankers/constant_category_ranker.h"
 #include "components/variations/variations_associated_data.h"
 
 namespace ntp_snippets {
 
 const base::Feature kArticleSuggestionsFeature{
     "NTPArticleSuggestions", base::FEATURE_ENABLED_BY_DEFAULT};
 
@@ skipping 69 matching lines @@
       return base::MakeUnique<ClickBasedCategoryRanker>(pref_service,
                                                         std::move(clock));
     default:
       NOTREACHED() << "The category ranker choice value is "
                    << static_cast<int>(choice);
   }
   return nullptr;
 }
 
 }  // namespace ntp_snippets