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

ui/native_theme/native_theme_aura.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_AURA_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_AURA_H_
 
 #include "base/macros.h"
 #include "ui/native_theme/native_theme_base.h"
 
 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:
   SkColor GetSystemColor(ColorId color_id) const override;
   void PaintMenuPopupBackground(
       SkCanvas* canvas,
       const gfx::Size& size,
       const MenuBackgroundExtraParams& menu_background) const override;
   void PaintMenuItemBackground(
       SkCanvas* canvas,
@@ skipping 15 matching lines @@
                            const gfx::Rect& rect,
                            ScrollbarOverlayColorTheme theme) const override;
   void PaintScrollbarCorner(SkCanvas* canvas,
                             State state,
                             const gfx::Rect& rect) const override;
   gfx::Size GetPartSize(Part part,
                         State state,
                         const ExtraParams& extra) const override;
 
  private:
+  bool use_overlay_scrollbars_;
+
   DISALLOW_COPY_AND_ASSIGN(NativeThemeAura);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_AURA_H_