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

ui/native_theme/native_theme_win.h

Index: ui/native_theme/native_theme_win.h
diff --git a/ui/native_theme/native_theme_win.h b/ui/native_theme/native_theme_win.h
index fa2c82ce608fe55a610904f8a99c81202d8c8b56..e8d4673aa1c0faa9c2ef08cce20acbef96b75180 100644
--- a/ui/native_theme/native_theme_win.h
+++ b/ui/native_theme/native_theme_win.h
@@ -91,9 +91,6 @@ class NATIVE_THEME_EXPORT NativeThemeWin : public NativeTheme,
   // Returns true if classic theme is in use.
   bool IsClassicTheme(ThemeName name) const;
 
-  // Gets our singleton instance.
-  static NativeThemeWin* instance();
-
   HRESULT PaintTextField(HDC hdc,
                          int part_id,
                          int state_id,
@@ -115,6 +112,10 @@ class NATIVE_THEME_EXPORT NativeThemeWin : public NativeTheme,
   SkColor GetSystemColor(ColorId color_id) const override;
 
  protected:
+  friend class NativeTheme;
+  // Gets our singleton instance.
+  static NativeThemeWin* instance();
+
   NativeThemeWin();
   ~NativeThemeWin() override;