Use nine-patch resource for drawing Aura overlay scrollbar thumb.

ui/native_theme/native_theme_win.h

Index: ui/native_theme/native_theme_win.h
diff --git a/ui/native_theme/native_theme_win.h b/ui/native_theme/native_theme_win.h
index 2f83a07b5e30bc03cb867307e60504bed5728f9f..d2585e41d057b1e9b1568898cf36a2295ba8c1d5 100644
--- a/ui/native_theme/native_theme_win.h
+++ b/ui/native_theme/native_theme_win.h
@@ -109,6 +109,10 @@ class NATIVE_THEME_EXPORT NativeThemeWin : public NativeTheme,
              const ExtraParams& extra) const override;
   SkColor GetSystemColor(ColorId color_id) const override;
 
+  bool SupportsNinePatch(Part part) const override;
+  gfx::Size GetNinePatchCanvasSize(Part part) const override;
+  gfx::Rect GetNinePatchAperture(Part part) const override;
+
  protected:
   friend class NativeTheme;
   // Gets our singleton instance.