Convert the overlay scrollbar flag to use FeatureList

ui/native_theme/BUILD.gn

 # 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.
 
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 
 component("native_theme") {
   sources = [
     "common_theme.cc",
     "common_theme.h",
     "native_theme.cc",
     "native_theme.h",
     "native_theme_android.cc",
     "native_theme_android.h",
     "native_theme_base.cc",
     "native_theme_base.h",
+    "native_theme_features.cc",
+    "native_theme_features.h",
     "native_theme_mac.h",
     "native_theme_mac.mm",
     "native_theme_observer.cc",
     "native_theme_observer.h",
-    "native_theme_switches.cc",
-    "native_theme_switches.h",
   ]
 
   if (use_aura) {
     sources += [
       "native_theme_aura.cc",
       "native_theme_aura.h",
       "native_theme_dark_aura.cc",
       "native_theme_dark_aura.h",
       "overlay_scrollbar_constants_aura.h",
     ]
@@ skipping 59 matching lines @@
   deps = [
     ":native_theme",
     "//base/test:run_all_unittests",
     "//base/test:test_support",
     "//skia",
     "//testing/gtest",
     "//ui/base",
     "//ui/gfx/geometry:geometry",
   ]
 }