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

components/ntp_snippets/features.h

Index: components/ntp_snippets/features.h
diff --git a/components/ntp_snippets/features.h b/components/ntp_snippets/features.h
index d4cf6a023d4a9398335fc392b3235c7aa52f524c..51e5fc0aef7ca11c171f4408ad6092fa3762d3eb 100644
--- a/components/ntp_snippets/features.h
+++ b/components/ntp_snippets/features.h
@@ -41,6 +41,23 @@ 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_