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

ui/native_theme/native_theme_win.h

 // Copyright (c) 2012 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_WIN_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_WIN_H_
 
 // A wrapper class for working with custom XP/Vista themes provided in
 // uxtheme.dll.  This is a singleton class that can be grabbed using
 // NativeThemeWin::instance().
@@ skipping 73 matching lines @@
   // consistent visual results.
   void DisableTheming() const;
 
   // Closes cached theme handles so we can unload the DLL or update our UI
   // for a theme change.
   void CloseHandles() const;
 
   // 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,
                          int classic_state,
                          RECT* rect,
                          COLORREF color,
                          bool fill_content_area,
                          bool draw_edges) const;
 
   // NativeTheme implementation:
   gfx::Size GetPartSize(Part part,
                         State state,
                         const ExtraParams& extra) const override;
   void Paint(SkCanvas* canvas,
              Part part,
              State state,
              const gfx::Rect& rect,
              const ExtraParams& extra) const override;
   SkColor GetSystemColor(ColorId color_id) const override;
 
  protected:
+  friend class NativeTheme;
+  // Gets our singleton instance.
+  static NativeThemeWin* instance();
+
   NativeThemeWin();
   ~NativeThemeWin() override;
 
  private:
   // gfx::SysColorChangeListener implementation:
   void OnSysColorChange() override;
 
   // Update the locally cached set of system colors.
   void UpdateSystemColors();
 
@@ skipping 243 matching lines @@
 
   // Is |is_using_high_contrast_| valid?
   mutable bool is_using_high_contrast_valid_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeThemeWin);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_WIN_H_