Fix undefined DesktopWindowTreeHost::GetNativeTheme for Ozone desktop

ui/views/widget/desktop_aura/desktop_window_tree_host_ozone.cc

Index: ui/views/widget/desktop_aura/desktop_window_tree_host_ozone.cc
diff --git a/ui/views/widget/desktop_aura/desktop_window_tree_host_ozone.cc b/ui/views/widget/desktop_aura/desktop_window_tree_host_ozone.cc
index 1a78a9ab57ef7a3f27f09a630285ccf685ac01db..e67af41ed0034fc6423b2c2cfd127f7e269f3437 100644
--- a/ui/views/widget/desktop_aura/desktop_window_tree_host_ozone.cc
+++ b/ui/views/widget/desktop_aura/desktop_window_tree_host_ozone.cc
@@ -3,6 +3,7 @@
 // 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"
 
@@ -17,4 +18,9 @@ DesktopWindowTreeHost* DesktopWindowTreeHost::Create(
                                          desktop_native_widget_aura);
 }
 
+// static
+ui::NativeTheme* DesktopWindowTreeHost::GetNativeTheme(aura::Window* window) {
+  return ui::NativeThemeAura::instance();
+}
+
 }  // namespace views