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

ui/native_theme/native_theme_android.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_ANDROID_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_ANDROID_H_
 
 #include "base/macros.h"
 #include "ui/native_theme/native_theme_base.h"
 
 namespace ui {
 
 // Android implementation of native theme support.
 class NativeThemeAndroid : public NativeThemeBase {
  public:
-  static NativeThemeAndroid* instance();
-
   // NativeThemeBase:
   gfx::Size GetPartSize(Part part,
                         State state,
                         const ExtraParams& extra) const override;
   SkColor GetSystemColor(ColorId color_id) const override;
 
  protected:
+  friend class NativeTheme;
+  static NativeThemeAndroid* instance();
+
   // NativeThemeBase:
   void AdjustCheckboxRadioRectForPadding(SkRect* rect) const override;
 
  private:
   NativeThemeAndroid();
   ~NativeThemeAndroid() override;
 
   DISALLOW_COPY_AND_ASSIGN(NativeThemeAndroid);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_ANDROID_H_