Manual revert of "Convert the overlay scrollbar flag to use FeatureList"

ui/native_theme/native_theme_features.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 "ui/native_theme/native_theme_features.h"
-
-namespace features {
-
-#if defined(OS_ANDROID) || defined(OS_CHROMEOS)
-constexpr base::FeatureState kOverlayScrollbarFeatureState =
-    base::FEATURE_ENABLED_BY_DEFAULT;
-#else
-constexpr base::FeatureState kOverlayScrollbarFeatureState =
-    base::FEATURE_DISABLED_BY_DEFAULT;
-#endif
-
-// Enables or disables overlay scrollbars in Blink (i.e. web content) on Aura
-// or Linux.  The status of native UI overlay scrollbars is determined in
-// PlatformStyle::CreateScrollBar. Does nothing on Mac.
-const base::Feature kOverlayScrollbar{"OverlayScrollbar",
-                                      kOverlayScrollbarFeatureState};
-
-}  // namespace features
-
-namespace ui {
-
-bool IsOverlayScrollbarEnabled() {
-  return base::FeatureList::IsEnabled(features::kOverlayScrollbar);
-}
-
-}  // namespace ui