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

ui/views/views_delegate.cc

 // Copyright (c) 2013 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/views/views_delegate.h"
 
 #include "base/command_line.h"
 #include "ui/views/views_touch_selection_controller_factory.h"
 #include "ui/views/widget/native_widget_private.h"
 
@@ skipping 102 matching lines @@
 int ViewsDelegate::GetAppbarAutohideEdges(HMONITOR monitor,
                                           const base::Closure& callback) {
   return EDGE_BOTTOM;
 }
 #endif
 
 scoped_refptr<base::TaskRunner> ViewsDelegate::GetBlockingPoolTaskRunner() {
   return nullptr;
 }
 
+ui::NativeTheme* ViewsDelegate::GetNativeThemeOverride(const Widget* widget) {
+  return nullptr;
+}
+
 ViewsDelegate::ViewsDelegate()
     : views_tsc_factory_(new ViewsTouchEditingControllerFactory) {
   DCHECK(!views_delegate);
   views_delegate = this;
 
   ui::TouchEditingControllerFactory::SetInstance(views_tsc_factory_.get());
 
 #if defined(USE_AURA)
   touch_selection_menu_runner_.reset(new TouchSelectionMenuRunnerViews());
 #endif
 }
 
 }  // namespace views