Revert 136996 - ui: Move NativeTheme files into ui/base/native_theme/ directory.

ui/base/native_theme/native_theme_gtk.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_BASE_NATIVE_THEME_NATIVE_THEME_GTK_H_
-#define UI_BASE_NATIVE_THEME_NATIVE_THEME_GTK_H_
-#pragma once
-
-#include "ui/base/native_theme/native_theme_base.h"
-
-namespace ui {
-
-// 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 ui
-
-#endif  // UI_BASE_NATIVE_THEME_NATIVE_THEME_GTK_H_