Convert the overlay scrollbar flag to use FeatureList

base/test/scoped_feature_list.h

Index: base/test/scoped_feature_list.h
diff --git a/base/test/scoped_feature_list.h b/base/test/scoped_feature_list.h
index 99e07f5374a96bf9668e9805da592cc9cd4e5805..d1e05baa441b18b8eeba5ec76f794cd8880b8456 100644
--- a/base/test/scoped_feature_list.h
+++ b/base/test/scoped_feature_list.h
@@ -47,6 +47,14 @@ class ScopedFeatureList final {
   // feature.
   void InitAndDisableFeature(const base::Feature& feature);
 
+  // Initializes and registers a FeatureList instance with
+  // override_enabled_features and override_disabled_features overrides
+  // feature_list.
+  void InitWithFeatureListAndOverrides(
+      base::FeatureList* feature_list,
+      const std::vector<std::string>& override_enabled_features,
+      const std::vector<std::string>& override_disabled_features);
+
  private:
   std::unique_ptr<FeatureList> original_feature_list_;