[NTP::SectionOrder] First version of click based category ranker.

components/ntp_snippets/pref_names.h

Index: components/ntp_snippets/pref_names.h
diff --git a/components/ntp_snippets/pref_names.h b/components/ntp_snippets/pref_names.h
index c0ddda05cd0eaab5604cf7e4034a803197cda1af..5ff759f0d6982a7817159e74ee36fd818272c93b 100644
--- a/components/ntp_snippets/pref_names.h
+++ b/components/ntp_snippets/pref_names.h
@@ -69,6 +69,9 @@ 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[];
+
 }  // namespace prefs
 }  // namespace ntp_snippets