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

chrome/browser/ntp_snippets/content_suggestions_service_factory.cc

Index: chrome/browser/ntp_snippets/content_suggestions_service_factory.cc
diff --git a/chrome/browser/ntp_snippets/content_suggestions_service_factory.cc b/chrome/browser/ntp_snippets/content_suggestions_service_factory.cc
index 2b4a13be546b13a67961c9f27359965406e461e5..44ea2695af77895b9eaeca3fb927d001dad9f465 100644
--- a/chrome/browser/ntp_snippets/content_suggestions_service_factory.cc
+++ b/chrome/browser/ntp_snippets/content_suggestions_service_factory.cc
@@ -29,6 +29,8 @@
 #include "components/keyed_service/content/browser_context_dependency_manager.h"
 #include "components/keyed_service/core/service_access_type.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"
@@ -45,6 +47,7 @@
 #include "components/signin/core/browser/profile_oauth2_token_service.h"
 #include "components/signin/core/browser/signin_manager.h"
 #include "components/translate/core/browser/language_model.h"
+#include "components/variations/variations_associated_data.h"
 #include "components/version_info/version_info.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/browser_thread.h"
@@ -75,6 +78,9 @@ using content::BrowserThread;
 using history::HistoryService;
 using image_fetcher::ImageFetcherImpl;
 using ntp_snippets::BookmarkSuggestionsProvider;
+using ntp_snippets::CategoryRanker;
+using ntp_snippets::ClickBasedCategoryRanker;
+using ntp_snippets::ConstantCategoryRanker;
 using ntp_snippets::ContentSuggestionsService;
 using ntp_snippets::ForeignSessionsSuggestionsProvider;
 using ntp_snippets::NTPSnippetsFetcher;
@@ -189,6 +195,26 @@ void RegisterForeignSessionsProvider(SyncService* sync_service,
   service->RegisterProvider(std::move(provider));
 }
 
+std::unique_ptr<CategoryRanker> BuildCategoryRanker(PrefService* pref_service) {
+  std::string category_ranker_parameter = variations::GetVariationParamValue(
+      ntp_snippets::kStudyName, "category_ranker");
+
+  if (category_ranker_parameter.empty()) {
+    // Default, Enabled or Disabled.
+    return base::MakeUnique<ConstantCategoryRanker>();
+  }
+  if (category_ranker_parameter == "constant") {
+    return base::MakeUnique<ConstantCategoryRanker>();
+  }
+  if (category_ranker_parameter == "click_based") {
+    return base::MakeUnique<ClickBasedCategoryRanker>(pref_service);
+  }
+
+  NOTREACHED() << "The category_ranker parameter value is "
+               << category_ranker_parameter;
+  return nullptr;
+}
+
 }  // namespace
 
 // static
@@ -243,8 +269,8 @@ KeyedService* ContentSuggestionsServiceFactory::BuildServiceInstanceFor(
   HistoryService* history_service = HistoryServiceFactory::GetForProfile(
       profile, ServiceAccessType::EXPLICIT_ACCESS);
   PrefService* pref_service = profile->GetPrefs();
-  auto category_ranker =
-      base::MakeUnique<ntp_snippets::ConstantCategoryRanker>();
+  std::unique_ptr<CategoryRanker> category_ranker =
+      BuildCategoryRanker(pref_service);
   auto* service =
       new ContentSuggestionsService(state, signin_manager, history_service,
                                     pref_service, std::move(category_ranker));