Reland of Clean up some NativeTheme code.

ui/native_theme/native_theme_mac.mm

Index: ui/native_theme/native_theme_mac.mm
diff --git a/ui/native_theme/native_theme_mac.mm b/ui/native_theme/native_theme_mac.mm
index b5434a627750ac2232e72a958069f54ecc635403..36a58dc1d810ba40479bf2b03a421c9b16c67f28 100644
--- a/ui/native_theme/native_theme_mac.mm
+++ b/ui/native_theme/native_theme_mac.mm
@@ -113,6 +113,11 @@ NativeTheme* NativeTheme::GetInstanceForWeb() {
 }
 
 // static
+NativeTheme* NativeTheme::GetInstanceForNativeUi() {
+  return NativeThemeMac::instance();
+}
+
+// static
 NativeThemeMac* NativeThemeMac::instance() {
   CR_DEFINE_STATIC_LOCAL(NativeThemeMac, s_native_theme, ());
   return &s_native_theme;