Paint state transition for overlay 9 patch scrollbars

ui/native_theme/native_theme.h

Index: ui/native_theme/native_theme.h
diff --git a/ui/native_theme/native_theme.h b/ui/native_theme/native_theme.h
index 76d65ce2ecd2aae2619c14f84578eb55bc2ae177..0d5748b8fd5b7cc941bd8929da6346cc3696556d 100644
--- a/ui/native_theme/native_theme.h
+++ b/ui/native_theme/native_theme.h
@@ -223,6 +223,15 @@ class NATIVE_THEME_EXPORT NativeTheme {
                      const gfx::Rect& rect,
                      const ExtraParams& extra) const = 0;
 
+  // Paint part during state transition, used for overlay scrollbar state
+  // transition animation.
+  virtual void PaintStateTransition(SkCanvas* canvas,
+                                    Part part,
+                                    State startState,
+                                    State endState,
+                                    double progress,
+                                    const gfx::Rect& rect) const { }
+
   // Supports theme specific colors.
   void SetScrollbarColors(unsigned inactive_color,
                           unsigned active_color,