Convert the overlay scrollbar flag to use FeatureList

ui/native_theme/native_theme_switches.cc

-// 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.
-
-#include "base/command_line.h"
-#include "ui/native_theme/native_theme_switches.h"
-
-namespace switches {
-
-// Enables or disables overlay scrollbars in Blink (i.e. web content) on Aura
-// or Linux.  The status of native UI overlay scrollbars are determined in
-// PlatformStyle::CreateScrollBar. Does nothing on Mac.
-const char kEnableOverlayScrollbar[] = "enable-overlay-scrollbar";
-const char kDisableOverlayScrollbar[] = "disable-overlay-scrollbar";
-
-}  // namespace switches
-
-namespace ui {
-
-bool IsOverlayScrollbarEnabled() {
-  const base::CommandLine& command_line =
-      *base::CommandLine::ForCurrentProcess();
-
-  if (command_line.HasSwitch(switches::kDisableOverlayScrollbar))
-    return false;
-
-#if defined(OS_CHROMEOS)
-  return true;
-#else
-  return command_line.HasSwitch(switches::kEnableOverlayScrollbar);
-#endif
-}
-
-}  // namespace ui