Add a new GetSystemColor method to native theme.

ui/gfx/native_theme_gtk.h

 // Copyright (c) 2011 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_GFX_NATIVE_THEME_GTK_H_
 #define UI_GFX_NATIVE_THEME_GTK_H_
 #pragma once
 
 #include "ui/gfx/native_theme_base.h"
 
 namespace gfx {
 
 // GTK implementation of native theme support.
 class NativeThemeGtk : public NativeThemeBase {
  public:
   static const NativeThemeGtk* instance();
 
+  virtual SkColor GetSystemColor(ColorId color_id) const OVERRIDE;
+
  private:
   NativeThemeGtk();
   virtual ~NativeThemeGtk();
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_NATIVE_THEME_GTK_H_