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

chrome/browser/about_flags.cc

Index: chrome/browser/about_flags.cc
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 4afb92b95251e881d00156464f22937759934ca7..a396e2bc06eb980871742725febb54331ebbb288 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -510,6 +510,29 @@ const FeatureEntry::Choice kNtpSwitchToExistingTabChoices[] = {
 #endif  // OS_ANDROID
 
 #if defined(OS_ANDROID)
+const FeatureEntry::FeatureParam
+    kNTPSnippetsCategoryRankerFeatureVariationConstant[] = {
+        {ntp_snippets::kCategoryRankerParameter,
+         ntp_snippets::kCategoryRankerConstantRanker},
+};
+
+const FeatureEntry::FeatureParam
+    kNTPSnippetsCategoryRankerFeatureVariationClickBased[] = {
+        {ntp_snippets::kCategoryRankerParameter,
+         ntp_snippets::kCategoryRankerClickBasedRanker},
+};
+
+const FeatureEntry::FeatureVariation
+    kNTPSnippetsCategoryRankerFeatureVariations[] = {
+        {"(constant)", kNTPSnippetsCategoryRankerFeatureVariationConstant,
+         arraysize(kNTPSnippetsCategoryRankerFeatureVariationConstant),
+         nullptr},
+        {"(click based)", kNTPSnippetsCategoryRankerFeatureVariationClickBased,
+         arraysize(kNTPSnippetsCategoryRankerFeatureVariationClickBased),
+         nullptr}};
+#endif  // OS_ANDROID
+
+#if defined(OS_ANDROID)
 const FeatureEntry::FeatureParam kNTPSnippetsFeatureVariationOnlyPersonal[] = {
     {"fetching_personalization", "personal"},
 };
@@ -1839,6 +1862,12 @@ const FeatureEntry kFeatureEntries[] = {
      SINGLE_VALUE_TYPE(extensions::switches::kDisableTabForDesktopShare)},
 #endif  // ENABLE_EXTENSIONS
 #if defined(OS_ANDROID)
+    {"ntp-category-ranker", IDS_FLAGS_NTP_CATEGORY_RANKER_NAME,
+     IDS_FLAGS_NTP_CATEGORY_RANKER_DESCRIPTION, kOsAndroid,
+     FEATURE_WITH_VARIATIONS_VALUE_TYPE(
+         ntp_snippets::kCategoryRanker,
+         kNTPSnippetsCategoryRankerFeatureVariations,
+         ntp_snippets::kStudyName)},
     {"enable-ntp-snippets", IDS_FLAGS_ENABLE_NTP_SNIPPETS_NAME,
      IDS_FLAGS_ENABLE_NTP_SNIPPETS_DESCRIPTION, kOsAndroid,
      FEATURE_WITH_VARIATIONS_VALUE_TYPE(