Change Aura overlay scrollbars from solid color to painted scrollbars.

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 85ce5bc708dbdfc1c4ad38697cca6aec0453a206..596552fb16e4a8763779059a8b5407a9b869fd54 100644
--- a/ui/native_theme/native_theme_aura.h
+++ b/ui/native_theme/native_theme_aura.h
@@ -53,6 +53,9 @@ class NATIVE_THEME_EXPORT NativeThemeAura : public NativeThemeBase {
                                           State endState,
                                           double progress,
                                           const gfx::Rect& rect) const override;
+  gfx::Size GetPartSize(Part part,
+                        State state,
+                        const ExtraParams& extra) const override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(NativeThemeAura);