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

ui/native_theme/native_theme_android.h

Index: ui/native_theme/native_theme_android.h
diff --git a/ui/native_theme/native_theme_android.h b/ui/native_theme/native_theme_android.h
index e504252a15d66fb82d60121943a3123136cc6f20..52b42e59b725516a920d731d7ed953de05a3ce23 100644
--- a/ui/native_theme/native_theme_android.h
+++ b/ui/native_theme/native_theme_android.h
@@ -13,8 +13,6 @@ namespace ui {
 // Android implementation of native theme support.
 class NativeThemeAndroid : public NativeThemeBase {
  public:
-  static NativeThemeAndroid* instance();
-
   // NativeThemeBase:
   gfx::Size GetPartSize(Part part,
                         State state,
@@ -22,6 +20,9 @@ class NativeThemeAndroid : public NativeThemeBase {
   SkColor GetSystemColor(ColorId color_id) const override;
 
  protected:
+  friend class NativeTheme;
+  static NativeThemeAndroid* instance();
+
   // NativeThemeBase:
   void AdjustCheckboxRadioRectForPadding(SkRect* rect) const override;