views: Make view::Views::GetPreferredSize() const.

ui/views/controls/scrollbar/kennedy_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_KENNEDY_SCROLL_BAR_H_
 #define UI_VIEWS_CONTROLS_SCROLLBAR_KENNEDY_SCROLL_BAR_H_
 
 #include "ui/views/controls/scrollbar/base_scroll_bar.h"
 
 namespace views {
 
 // The scrollbar of kennedy style. Transparent track and grey rectangle
 // thumb. Right now it doesn't have the way to share the background,
 // so it will accept the background color instead.
 class VIEWS_EXPORT KennedyScrollBar : public BaseScrollBar {
  public:
   explicit KennedyScrollBar(bool horizontal);
   virtual ~KennedyScrollBar();
 
  protected:
   // BaseScrollBar overrides:
   virtual gfx::Rect GetTrackBounds() const OVERRIDE;
 
   // ScrollBar overrides:
   virtual int GetLayoutSize() const OVERRIDE;
 
   // View overrides:
-  virtual gfx::Size GetPreferredSize() OVERRIDE;
+  virtual gfx::Size GetPreferredSize() const OVERRIDE;
   virtual void Layout() OVERRIDE;
   virtual void OnPaint(gfx::Canvas* canvas) OVERRIDE;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(KennedyScrollBar);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_CONTROLS_SCROLLBAR_KENNEDY_SCROLL_BAR_H_