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

ios/chrome/browser/ntp_snippets/ios_chrome_content_suggestions_service_factory.cc

Index: ios/chrome/browser/ntp_snippets/ios_chrome_content_suggestions_service_factory.cc
diff --git a/ios/chrome/browser/ntp_snippets/ios_chrome_content_suggestions_service_factory.cc b/ios/chrome/browser/ntp_snippets/ios_chrome_content_suggestions_service_factory.cc
index 232edcbd2c8c6658bae5de951e370b85f95ae812..354a69a7cd711cab4e0a8696fcc1b84b4b256f18 100644
--- a/ios/chrome/browser/ntp_snippets/ios_chrome_content_suggestions_service_factory.cc
+++ b/ios/chrome/browser/ntp_snippets/ios_chrome_content_suggestions_service_factory.cc
@@ -19,6 +19,7 @@
 #include "components/keyed_service/ios/browser_state_dependency_manager.h"
 #include "components/ntp_snippets/bookmarks/bookmark_suggestions_provider.h"
 #include "components/ntp_snippets/category_rankers/category_ranker.h"
+#include "components/ntp_snippets/category_rankers/click_based_category_ranker.h"
 #include "components/ntp_snippets/category_rankers/constant_category_ranker.h"
 #include "components/ntp_snippets/content_suggestions_service.h"
 #include "components/ntp_snippets/features.h"
@@ -75,6 +76,23 @@ void ParseJson(const std::string& json,
   }
 }
 
+std::unique_ptr<ntp_snippets::CategoryRanker> BuildCategoryRanker(
+    PrefService* pref_service) {
+  ntp_snippets::CategoryRankerChoice choice =
+      ntp_snippets::GetSelectedCategoryRanker();
+  switch (choice) {
+    case ntp_snippets::CategoryRankerChoice::CONSTANT:
+      return base::MakeUnique<ntp_snippets::ConstantCategoryRanker>();
+    case ntp_snippets::CategoryRankerChoice::CLICK_BASED:
+      return base::MakeUnique<ntp_snippets::ClickBasedCategoryRanker>(
+          pref_service);
+    default:
+      NOTREACHED() << "The category ranker choice value is "
+                   << static_cast<int>(choice);
+  }
+  return nullptr;
+}
+
 }  // namespace
 
 // static
@@ -126,7 +144,7 @@ IOSChromeContentSuggestionsServiceFactory::BuildServiceInstanceFor(
       ios::HistoryServiceFactory::GetForBrowserState(
           chrome_browser_state, ServiceAccessType::EXPLICIT_ACCESS);
   std::unique_ptr<ntp_snippets::CategoryRanker> category_ranker =
-      base::MakeUnique<ntp_snippets::ConstantCategoryRanker>();
+      BuildCategoryRanker(prefs);
   std::unique_ptr<ContentSuggestionsService> service =
       base::MakeUnique<ContentSuggestionsService>(state, signin_manager,
                                                   history_service, prefs,