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)

[dgozman, 2017/04/13 17:33:28]

I didn't find where this feature was enabled by default on ChromeOS before. Am I
missing something?

[chaopeng, 2017/04/13 19:45:32]

The CrOS enable by default is in here before.
https://cs.chromium.org/chromium/src/ui/native_theme/native_theme_switches.cc...

+const base::FeatureState kOverlayScrollbarFeatureState =

[Peter Kasting, 2017/04/13 18:28:15]

Nit: Prefer constexpr to const for compile-time constants.

+    base::FEATURE_ENABLED_BY_DEFAULT;
+#else
+const 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 are determined in
+// PlatformStyle::CreateScrollBar. Does nothing on Mac.
+const base::Feature kOverlayScrollbar{"OverlayScrollbar",
+                                      kOverlayScrollbarFeatureState};
+}  // namespace features

[Peter Kasting, 2017/04/13 18:28:15]

Nit: Blank line above?

+
+namespace ui {
+
+bool IsOverlayScrollbarEnabled() {
+  return base::FeatureList::IsEnabled(features::kOverlayScrollbar);
+}
+
+}  // namespace ui