[MD] Implement incognito colors as a NativeTheme (for Aura).

ui/native_theme/native_theme_win_dark.cc

+// Copyright 2015 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/native_theme/native_theme_win_dark.h"
+
+#include "ui/base/resource/material_design/material_design_controller.h"
+#include "ui/native_theme/native_theme_aura_dark.h"
+
+namespace ui {
+
+NativeThemeWinDark* NativeThemeWinDark::instance() {
+  CR_DEFINE_STATIC_LOCAL(NativeThemeWinDark, s_native_theme, ());
+  return &s_native_theme;
+}
+
+NativeThemeWinDark::NativeThemeWinDark() {}
+
+NativeThemeWinDark::~NativeThemeWinDark() {}
+
+SkColor NativeThemeWinDark::GetSystemColor(ColorId color_id) const {
+  if (!ui::MaterialDesignController::IsModeMaterial())
+    return NativeThemeWin::GetSystemColor(color_id);
+
+  return NativeThemeAuraDark::instance()->GetSystemColor(color_id);
+}
+
+}  // namespace ui