MacViews: Implement scrollbar theming.

ui/native_theme/native_theme_base.h

 // Copyright (c) 2012 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_NATIVE_THEME_NATIVE_THEME_BASE_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_BASE_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 182 matching lines @@
 
   // Paint the common parts of the checkboxes and radio buttons.
   // borderRadius specifies how rounded the corners should be.
   SkRect PaintCheckboxRadioCommon(
       SkCanvas* canvas,
       State state,
       const gfx::Rect& rect,
       const SkScalar borderRadius) const;
 
   unsigned int scrollbar_width_;
+
+  // The length of the arrow buttons, 0 means no buttons are drawn.
   unsigned int scrollbar_button_length_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeThemeBase);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_BASE_H_