[NTP::SectionOrder] Implement click counts decay.

components/ntp_snippets/pref_names.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_PREF_NAMES_H_
 #define COMPONENTS_NTP_SNIPPETS_PREF_NAMES_H_
 
 namespace ntp_snippets {
 namespace prefs {
 
@@ skipping 53 matching lines @@
 
 // The pref name for the last time a new NTP was opened.
 extern const char kUserClassifierLastTimeToOpenNTP[];
 // The pref name for the last time content suggestions were shown to the user.
 extern const char kUserClassifierLastTimeToShowSuggestions[];
 // The pref name for the last time content suggestions were used by the user.
 extern const char kUserClassifierLastTimeToUseSuggestions[];
 
 // The pref name for the current order of categories and their clicks.
 extern const char kClickBasedCategoryRankerOrderWithClicks[];
+// The pref name for the time when last click decay has happened.
+extern const char kClickBasedCategoryRankerLastDecayTime[];
 
 }  // namespace prefs
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_PREF_NAMES_H_