Use the default theme as the fallback UI in Blink

ui/native_theme/native_theme_aura.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_AURA_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_AURA_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "ui/native_theme/native_theme_base.h"
 
 namespace ui {
 
 // Aura implementation of native theme support.
 class NATIVE_THEME_EXPORT NativeThemeAura : public NativeThemeBase {
  public:
   static NativeThemeAura* instance();
 
  private:
   NativeThemeAura();
   virtual ~NativeThemeAura();
 
   // Overridden from NativeThemeBase:
-  virtual SkColor GetSystemColor(ColorId color_id) const OVERRIDE;
   virtual void PaintMenuPopupBackground(
       SkCanvas* canvas,
       const gfx::Size& size,
       const MenuBackgroundExtraParams& menu_background) const OVERRIDE;
   virtual void PaintScrollbarTrack(
       SkCanvas* canvas,
       Part part,
       State state,
       const ScrollbarTrackExtraParams& extra_params,
       const gfx::Rect& rect) const OVERRIDE;
   virtual void PaintScrollbarThumb(SkCanvas* canvas,
                                    Part part,
                                    State state,
                                    const gfx::Rect& rect) const OVERRIDE;
 
   DISALLOW_COPY_AND_ASSIGN(NativeThemeAura);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_AURA_H_