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

ui/native_theme/native_theme_mac.h

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_NATIVE_THEME_NATIVE_THEME_MAC_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_MAC_H_
 
 #include "base/macros.h"
 #include "ui/native_theme/native_theme_base.h"
 #include "ui/native_theme/native_theme_export.h"
 
 namespace ui {
 
 // Mac implementation of native theme support.
 class NATIVE_THEME_EXPORT NativeThemeMac : public NativeThemeBase {
  public:
   static const int kButtonCornerRadius = 3;
 
   // Type of gradient to use on a button background. Use HIGHLIGHTED for the
   // default button of a window and all combobox controls, but only when the
   // window is active.
   enum class ButtonBackgroundType {
     DISABLED,
     HIGHLIGHTED,
     NORMAL,
     PRESSED,
     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.
   static SkColor ApplySystemControlTint(SkColor color);
 
   // Overridden from NativeTheme:
   SkColor GetSystemColor(ColorId color_id) const override;
 
   // Overridden from NativeThemeBase:
   void PaintMenuPopupBackground(
@@ skipping 19 matching lines @@
   // style is used for dialog buttons, comboboxes, and tabbed pane tabs.
   // Depending on the control part being drawn, the left or the right side can
   // be given rounded corners.
   static void PaintStyledGradientButton(SkCanvas* canvas,
                                         const gfx::Rect& bounds,
                                         ButtonBackgroundType type,
                                         bool round_left,
                                         bool round_right,
                                         bool focus);
 
+ protected:
+  friend class NativeTheme;
+  static NativeThemeMac* instance();
+
  private:
   NativeThemeMac();
   ~NativeThemeMac() override;
 
   DISALLOW_COPY_AND_ASSIGN(NativeThemeMac);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_MAC_H_