[NTP::SectionOrder] Add a flag to choose category ranker.

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 <string>
 
 #include "base/feature_list.h"
@@ skipping 23 matching lines @@
 
 // Feature to allow UI as specified here: https://crbug.com/660837.
 extern const base::Feature kIncreasedVisibility;
 
 // Feature to enable the Fetch More action
 extern const base::Feature kFetchMoreFeature;
 
 // Feature to prefer AMP URLs over regular URLs when available.
 extern const base::Feature kPreferAmpUrlsFeature;
 
+// Feature to choose a category ranker.
+extern const base::Feature kCategoryRanker;
+
+// Parameter for a kCategoryRanker feature flag.
+extern const char kCategoryRankerParameter[];
+// Possible values of the parameter above.
+extern const char kCategoryRankerConstantRanker[];
+extern const char kCategoryRankerClickBasedRanker[];
+
+enum class CategoryRankerChoice {
+  CONSTANT,
+  CLICK_BASED,
+};
+
+// Returns which CategoryRanker to use according to kCategoryRanker feature.
+CategoryRankerChoice GetSelectedCategoryRanker();
+
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_FEATURES_H_