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

ui/native_theme/native_theme_mac.h

Index: ui/native_theme/native_theme_mac.h
diff --git a/ui/native_theme/native_theme_mac.h b/ui/native_theme/native_theme_mac.h
index ec48119d26442030155d1a14ee8235f45d4becfb..8f870ea00b915981694a0d337b4b4a22531edfae 100644
--- a/ui/native_theme/native_theme_mac.h
+++ b/ui/native_theme/native_theme_mac.h
@@ -27,8 +27,6 @@ class NATIVE_THEME_EXPORT NativeThemeMac : public NativeThemeBase {
     COUNT
   };
 
-  static NativeThemeMac* instance();
-
   // Adjusts an SkColor based on the current system control tint. For example,
   // if the current tint is "graphite", this function maps the provided value to
   // an appropriate gray.
@@ -68,6 +66,10 @@ class NATIVE_THEME_EXPORT NativeThemeMac : public NativeThemeBase {
                                         bool round_right,
                                         bool focus);
 
+ protected:
+  friend class NativeTheme;
+  static NativeThemeMac* instance();
+
  private:
   NativeThemeMac();
   ~NativeThemeMac() override;