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

components/ntp_snippets/features.cc

 // 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.
 
 #include "components/ntp_snippets/features.h"
 
 #include "components/variations/variations_associated_data.h"
 
 namespace ntp_snippets {
 
@@ skipping 26 matching lines @@
 
 const base::Feature kForeignSessionsSuggestionsFeature{
     "NTPForeignSessionsSuggestions", base::FEATURE_DISABLED_BY_DEFAULT};
 
 const base::Feature kFetchMoreFeature{"NTPSuggestionsFetchMore",
                                       base::FEATURE_ENABLED_BY_DEFAULT};
 
 const base::Feature kPreferAmpUrlsFeature{"NTPPreferAmpUrls",
                                           base::FEATURE_ENABLED_BY_DEFAULT};
 
+const base::Feature kCategoryRanker{"NTPCategoryRanker",
+                                    base::FEATURE_ENABLED_BY_DEFAULT};
+
+CategoryRankerChoice GetSelectedCategoryRanker() {
+  std::string category_ranker_value =
+      variations::GetVariationParamValueByFeature(kCategoryRanker,
+                                                  kCategoryRankerParameter);
+
+  if (category_ranker_value.empty()) {
+    // Default, Enabled or Disabled.
+    return CategoryRankerChoice::CONSTANT;
+  }
+  if (category_ranker_value == kCategoryRankerConstantRanker) {
+    return CategoryRankerChoice::CONSTANT;
+  }
+  if (category_ranker_value == kCategoryRankerClickBasedRanker) {
+    return CategoryRankerChoice::CLICK_BASED;
+  }
+
+  NOTREACHED() << "The " << kCategoryRankerParameter << " parameter value is '"
+               << category_ranker_value << "'";
+  return CategoryRankerChoice::CONSTANT;
+}
+
 }  // namespace ntp_snippets