[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 d065de4d250d36d8efd9be9dead54f0f974f21d7..2d7ea7ac1ae49d57ce58c7cbf2c6820869e8a1e9 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"
@@ -46,6 +48,7 @@
 #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"
@@ -76,6 +79,9 @@ 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;
@@ -198,6 +204,26 @@ 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(

[jkrcal, 2016/12/22 10:06:01]

Can you use variations::GetVariationParamValueByFeatureName?

We try to move reading all variation params to this way as it allows us to split
the variation configs into multiple files (multiple studies).

[vitaliii, 2016/12/22 13:11:37]

Done.

+      ntp_snippets::kStudyName, "category_ranker");
+
+  if (category_ranker_parameter.empty()) {
+    // Default, Enabled or Disabled.
+    return base::MakeUnique<ConstantCategoryRanker>();
+  }
+  if (category_ranker_parameter == "constant") {

[jkrcal, 2016/12/22 10:06:01]

Do we need this option? I would have just 
 - Default    = constant, 
 - Enabled    = constant, 
 - Enabled (click based), 
 - Disabled    = constant, bit unfortunately

[tschumann, 2016/12/22 10:31:26]

i like the idea of using the strings in less places and an enum instead. in that
case, the default semantics would also live in a single place (where the strings
get turned into the enum) and here, we could have a simple switch over the value
and no need to interpret defaults etc.

[vitaliii, 2016/12/22 13:11:37]

I think - yes.

1) I treat all of "Default", "Enabled" and "Disabled" as default ranker, so it
is nice to have an option to choose constant ranker once default is changed to
something else.
2) If one is not sure which ranker is considered default, one can explicitly
force constant ranker if needed.

[vitaliii, 2016/12/22 13:11:37]

Done.

[jkrcal, 2016/12/22 13:39:15]

Makes sense. Ack.

+    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
@@ -252,8 +278,8 @@ 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));