Revert of Clean up some NativeTheme code.

ui/native_theme/native_theme_win.cc

Index: ui/native_theme/native_theme_win.cc
diff --git a/ui/native_theme/native_theme_win.cc b/ui/native_theme/native_theme_win.cc
index 7d4f0798c675d2c8caad40510a22fa71c1c2b486..cc90f3e2060515fbbc6dab55763fcec1f0a1763d 100644
--- a/ui/native_theme/native_theme_win.cc
+++ b/ui/native_theme/native_theme_win.cc
@@ -149,10 +149,6 @@
 }  // namespace
 
 namespace ui {
-
-NativeTheme* NativeTheme::GetInstanceForNativeUi() {
-  return NativeThemeWin::instance();
-}
 
 bool NativeThemeWin::IsThemingActive() const {
   return is_theme_active_ && is_theme_active_();