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 are determined in
+// PlatformStyle::CreateScrollBar. Does nothing on Mac.
+constexpr base::Feature kOverlayScrollbar{"OverlayScrollbar",

[dcheng, 2017/04/14 23:30:17]

This is declared as extern const in the header, but constexpr here? Does that
work? IIRC it's non-trivial to get an extern constexpr object... so what's
actually happening here?

[chaopeng, 2017/04/15 00:12:54]

It works. I just follow the declaration from this file and it seems we use this
for every feature declaration.

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