Use overlay scrollbars for the cros system menu.

ash/common/system/tray/fixed_sized_scroll_view.h

 // Copyright 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 ASH_COMMON_SYSTEM_TRAY_FIXED_SIZED_SCROLL_VIEW_H_
 #define ASH_COMMON_SYSTEM_TRAY_FIXED_SIZED_SCROLL_VIEW_H_
 
 #include "base/macros.h"
 #include "ui/views/controls/scroll_view.h"
 
 namespace ash {
 
 // A custom scroll-view that has a specified dimension.
+// TODO(estade): can we get rid of this class? In MD, it hardly differs in
+// behavior from ScrollView.
 class FixedSizedScrollView : public views::ScrollView {
  public:
   FixedSizedScrollView();
   ~FixedSizedScrollView() override;
 
   void SetContentsView(views::View* view);
 
   // Change the fixed size of the view. Invalidates the layout (by calling
   // PreferredSizeChanged()).
   void SetFixedSize(const gfx::Size& size);
 
-  void set_fixed_size(const gfx::Size& size) { fixed_size_ = size; }
+  void set_fixed_size(const gfx::Size& size);
 
   // Overridden from views::View:
   gfx::Size GetPreferredSize() const override;
   void Layout() override;
 
  protected:
   // Overridden from views::View:
   void OnBoundsChanged(const gfx::Rect& previous_bounds) override;
 
  private:
   gfx::Size fixed_size_;
 
   DISALLOW_COPY_AND_ASSIGN(FixedSizedScrollView);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_SYSTEM_TRAY_FIXED_SIZED_SCROLL_VIEW_H_