[NTP::SectionOrder] Implement click counts decay.

components/ntp_snippets/features.h

 // 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.
 
 #ifndef COMPONENTS_NTP_SNIPPETS_FEATURES_H_
 #define COMPONENTS_NTP_SNIPPETS_FEATURES_H_
 
 #include <memory>
 #include <string>
 
 #include "base/feature_list.h"
 #include "components/ntp_snippets/category_rankers/category_ranker.h"
 #include "components/prefs/pref_service.h"
 
+namespace base {
+class Clock;
+}
+
 namespace ntp_snippets {
 
 // Features to turn individual providers/categories on/off.
 extern const base::Feature kArticleSuggestionsFeature;
 extern const base::Feature kBookmarkSuggestionsFeature;
 extern const base::Feature kRecentOfflineTabSuggestionsFeature;
 extern const base::Feature kPhysicalWebPageSuggestionsFeature;
 extern const base::Feature kForeignSessionsSuggestionsFeature;;
 
 // Feature to allow the 'save to offline' option to appear in the snippets
@@ skipping 31 matching lines @@
 enum class CategoryRankerChoice {
   CONSTANT,
   CLICK_BASED,
 };
 
 // Returns which CategoryRanker to use according to kCategoryRanker feature.
 CategoryRankerChoice GetSelectedCategoryRanker();
 
 // Builds a CategoryRanker according to kCategoryRanker feature.
 std::unique_ptr<CategoryRanker> BuildSelectedCategoryRanker(
-    PrefService* pref_service);
+    PrefService* pref_service,
+    std::unique_ptr<base::Clock> clock);
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_FEATURES_H_