Convert the overlay scrollbar flag to use FeatureList

ui/native_theme/native_theme_features.h

Index: ui/native_theme/native_theme_features.h
diff --git a/ui/native_theme/native_theme_switches.h b/ui/native_theme/native_theme_features.h
similarity index 75%
rename from ui/native_theme/native_theme_switches.h
rename to ui/native_theme/native_theme_features.h
index 026ee70eca378be31944d0b8c56fd8d6d96583fc..9b8956bd2e33dbb44a1ed70808fca1ce65523f8f 100644
--- a/ui/native_theme/native_theme_switches.h
+++ b/ui/native_theme/native_theme_features.h
@@ -7,14 +7,14 @@
 #ifndef UI_NATIVE_THEME_NATIVE_THEME_SWITCHES_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_SWITCHES_H_
 
+#include "base/feature_list.h"
 #include "ui/native_theme/native_theme_export.h"
 
-namespace switches {
+namespace features {
 
-NATIVE_THEME_EXPORT extern const char kDisableOverlayScrollbar[];
-NATIVE_THEME_EXPORT extern const char kEnableOverlayScrollbar[];
+NATIVE_THEME_EXPORT extern const base::Feature kOverlayScrollbar;
 
-}  // namespace switches
+}  // namespace features
 
 namespace ui {