Fix undefined DesktopWindowTreeHost::GetNativeTheme for Ozone desktop

ui/views/widget/desktop_aura/desktop_window_tree_host_ozone.cc

 // Copyright 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/aura/window_tree_host.h"
+#include "ui/native_theme/native_theme_aura.h"
 #include "ui/views/widget/desktop_aura/desktop_factory_ozone.h"
 #include "ui/views/widget/desktop_aura/desktop_window_tree_host.h"
 
 namespace views {
 
 DesktopWindowTreeHost* DesktopWindowTreeHost::Create(
     internal::NativeWidgetDelegate* native_widget_delegate,
     DesktopNativeWidgetAura* desktop_native_widget_aura) {
   DesktopFactoryOzone* d_factory = DesktopFactoryOzone::GetInstance();
 
   return d_factory->CreateWindowTreeHost(native_widget_delegate,
                                          desktop_native_widget_aura);
 }
 
+// static
+ui::NativeTheme* DesktopWindowTreeHost::GetNativeTheme(aura::Window* window) {
+  return ui::NativeThemeAura::instance();
+}
+
 }  // namespace views