Try to get rid of all direct references to particular native themes.

ui/native_theme/native_theme.h

Index: ui/native_theme/native_theme.h
diff --git a/ui/native_theme/native_theme.h b/ui/native_theme/native_theme.h
index 5ed382a0a38df7e1e0dd8ec2927189aa2e0e9a7e..27b7832856739923ced34889e4014cbe342d3bfa 100644
--- a/ui/native_theme/native_theme.h
+++ b/ui/native_theme/native_theme.h
@@ -363,6 +363,9 @@ class NATIVE_THEME_EXPORT NativeTheme {
   // returning the port's subclass.
   static NativeTheme* GetInstanceForWeb();
 
+  // Returns a shared instance of the default native theme for native UI.
+  static NativeTheme* GetInstanceForNativeUi();
+
   // Add or remove observers to be notified when the native theme changes.
   void AddObserver(NativeThemeObserver* observer);
   void RemoveObserver(NativeThemeObserver* observer);