Convert the overlay scrollbar flag to use FeatureList

ui/native_theme/native_theme_features.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Defines all the command-line switches used by native theme
 
-#ifndef UI_NATIVE_THEME_NATIVE_THEME_SWITCHES_H_
-#define UI_NATIVE_THEME_NATIVE_THEME_SWITCHES_H_
+#ifndef UI_NATIVE_THEME_NATIVE_THEME_FEATURES_H_
+#define UI_NATIVE_THEME_NATIVE_THEME_FEATURES_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 {
 
 NATIVE_THEME_EXPORT bool IsOverlayScrollbarEnabled();
 
 }  // namespace ui
 
-#endif  // UI_NATIVE_THEME_NATIVE_THEME_SWITCHES_H_
+#endif  // UI_NATIVE_THEME_NATIVE_THEME_FEATURES_H_