ui: Move NativeTheme files into ui/base/native_theme/ directory.

ui/gfx/native_theme.cc

-// 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.
-
-#include "ui/gfx/native_theme.h"
-
-#include "ui/gfx/size.h"
-
-namespace gfx {
-
-unsigned int NativeTheme::thumb_inactive_color_ = 0xeaeaea;
-unsigned int NativeTheme::thumb_active_color_ = 0xf4f4f4;
-unsigned int NativeTheme::track_color_ = 0xd3d3d3;
-
-void NativeTheme::SetScrollbarColors(unsigned inactive_color,
-                                     unsigned active_color,
-                                     unsigned track_color) const {
-  thumb_inactive_color_ = inactive_color;
-  thumb_active_color_ = active_color;
-  track_color_ = track_color;
-}
-
-// NativeTheme::instance() is implemented in the platform specific source files,
-// such as native_theme_win.cc or native_theme_linux.cc
-
-}  // namespace gfx