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

ui/native_theme/native_theme_base.h

Index: ui/native_theme/native_theme_base.h
diff --git a/ui/native_theme/native_theme_base.h b/ui/native_theme/native_theme_base.h
index f47129381cdccca194daad59841cb7d2bf12029d..fa930bf7132449e77a8bdeb0231d358dfc4e062e 100644
--- a/ui/native_theme/native_theme_base.h
+++ b/ui/native_theme/native_theme_base.h
@@ -33,6 +33,10 @@ class NATIVE_THEME_EXPORT NativeThemeBase : public NativeTheme {
              const gfx::Rect& rect,
              const ExtraParams& extra) const override;
 
+  bool SupportsNinePatch(Part part) const override;
+  gfx::Size GetNinePatchCanvasSize(Part part) const override;
+  gfx::Rect GetNinePatchAperture(Part part) const override;
+
  protected:
   NativeThemeBase();
   ~NativeThemeBase() override;