Introduces a new scrollbar for message_center.

ui/views/controls/scrollbar/overlay_scroll_bar.h

+// Copyright (c) 2013 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.
+
+#ifndef UI_VIEWS_CONTROLS_SCROLLBAR_OVERLAY_SCROLL_BAR_H_
+#define UI_VIEWS_CONTROLS_SCROLLBAR_OVERLAY_SCROLL_BAR_H_
+
+#include "ui/base/animation/slide_animation.h"
+#include "ui/views/controls/scrollbar/base_scroll_bar.h"
+
+namespace views {
+
+// The transparent scrollbar which overlays its contents.
+class VIEWS_EXPORT OverlayScrollBar : public BaseScrollBar {
+ public:
+  explicit OverlayScrollBar(bool horizontal);
+  virtual ~OverlayScrollBar();
+
+ protected:
+  // BaseScrollBar overrides:
+  virtual gfx::Rect GetTrackBounds() const OVERRIDE;
+
+  // ScrollBar overrides:
+  virtual int GetLayoutSize() const OVERRIDE;
+  virtual int GetContentOverlapSize() const OVERRIDE;
+  virtual void OnMouseEnteredScrollView(const ui::MouseEvent& event) OVERRIDE;
+  virtual void OnMouseExitedScrollView(const ui::MouseEvent& event) OVERRIDE;
+
+  // View overrides:
+  virtual gfx::Size GetPreferredSize() OVERRIDE;
+  virtual void Layout() OVERRIDE;
+  virtual void OnPaint(gfx::Canvas* canvas) OVERRIDE;
+
+ private:
+  ui::SlideAnimation animation_;
+  DISALLOW_COPY_AND_ASSIGN(OverlayScrollBar);
+};
+
+}  // namespace views
+
+#endif  // UI_VIEWS_CONTROLS_SCROLLBAR_OVERLAY_SCROLL_BAR_H_