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 {
+
+// 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.
+#if defined(OS_ANDROID) || defined(OS_CHROMEOS)
+const base::Feature kOverlayScrollbar{"OverlayScrollbar",

[dtapuska, 2017/04/12 17:21:02]

I think it is probably more consistent to define
OVERLAY_SCROLLBAR_ENABLED_BY_DEFAULT but keep set it inside the define. But keey
the definition of base::Feature kOverlayScrollbar out.. Understand?

[chaopeng, 2017/04/12 20:23:15]

Done.

+                                      base::FEATURE_ENABLED_BY_DEFAULT};
+#else
+const base::Feature kOverlayScrollbar{"OverlayScrollbar",
+                                      base::FEATURE_DISABLED_BY_DEFAULT};
+#endif
+}  // namespace features
+
+namespace ui {
+
+bool IsOverlayScrollbarEnabled() {
+  return base::FeatureList::IsEnabled(features::kOverlayScrollbar);
+}
+
+}  // namespace ui