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

ui/native_theme/native_theme_aura.h

Index: ui/native_theme/native_theme_aura.h
diff --git a/ui/native_theme/native_theme_aura.h b/ui/native_theme/native_theme_aura.h
index e2fbfc4e741f42ac764795f448c36849b3579edd..347327ff22a1779ce8be143cf1124a7fea4b9971 100644
--- a/ui/native_theme/native_theme_aura.h
+++ b/ui/native_theme/native_theme_aura.h
@@ -12,11 +12,12 @@ namespace ui {
 
 // Aura implementation of native theme support.
 class NATIVE_THEME_EXPORT NativeThemeAura : public NativeThemeBase {
- public:
+ protected:
+  friend NativeTheme;
   static NativeThemeAura* instance();
+  static NativeThemeAura* web_instance();
 
- protected:
-  NativeThemeAura();
+  explicit NativeThemeAura(bool use_overlay_scrollbars);
   ~NativeThemeAura() override;
 
   // Overridden from NativeThemeBase:
@@ -52,6 +53,8 @@ class NATIVE_THEME_EXPORT NativeThemeAura : public NativeThemeBase {
                         const ExtraParams& extra) const override;
 
  private:
+  bool use_overlay_scrollbars_;
+
   DISALLOW_COPY_AND_ASSIGN(NativeThemeAura);
 };