Add base::FeatureParam<> struct

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 "base/metrics/field_trial_params.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.
@@ skipping 10 matching lines @@
 // 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;
 
 // Feature to allow the new Google favicon server for fetching publisher icons.
 extern const base::Feature kPublisherFaviconsFromNewServerFeature;
 
 // Parameter and its values for the kCategoryRanker feature flag.
-extern const char kCategoryRankerParameter[];
-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();
+extern const char kCategoryRankerConstantRanker[];
+extern const char kCategoryRankerClickBasedRanker[];
+extern const base::FeatureParam<CategoryRankerChoice> kCategoryRankerParameter;
 
 // Builds a CategoryRanker according to kCategoryRanker feature.
 std::unique_ptr<CategoryRanker> BuildSelectedCategoryRanker(
     PrefService* pref_service,
     std::unique_ptr<base::Clock> clock);
 
 // Feature to choose a default category order.
 extern const base::Feature kCategoryOrder;
 
 // Parameter and its values for the kCategoryOrder feature flag.
-extern const char kCategoryOrderParameter[];
-extern const char kCategoryOrderGeneral[];
-extern const char kCategoryOrderEmergingMarketsOriented[];
-
 enum class CategoryOrderChoice {
   GENERAL,
   EMERGING_MARKETS_ORIENTED,
 };
+extern const char kCategoryOrderGeneral[];
+extern const char kCategoryOrderEmergingMarketsOriented[];
+extern const base::FeatureParam<CategoryOrderChoice> kCategoryOrderParameter;
 
 // Returns which category order to use according to kCategoryOrder feature.
 CategoryOrderChoice GetSelectedCategoryOrder();
 
 }  // namespace ntp_snippets
 
 #endif  // COMPONENTS_NTP_SNIPPETS_FEATURES_H_