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

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 5f7a9d9ba5d82d55801f647f807d92fc3a029a87..31e86b704b90281a94791553c519c456cdb10e13 100644
--- a/ui/native_theme/native_theme.h
+++ b/ui/native_theme/native_theme.h
@@ -9,16 +9,13 @@
 #include "base/observer_list.h"
 #include "build/build_config.h"
 #include "third_party/skia/include/core/SkColor.h"
+#include "ui/gfx/geometry/rect.h"
+#include "ui/gfx/geometry/size.h"
 #include "ui/gfx/native_widget_types.h"
 #include "ui/native_theme/native_theme_export.h"
 
 class SkCanvas;
 
-namespace gfx {
-class Rect;
-class Size;
-}
-
 namespace ui {
 
 class NativeThemeObserver;
@@ -234,15 +231,9 @@ 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,
-                                    ScrollbarOverlayColorTheme theme) const {}
+  virtual bool SupportsNinePatch(Part part) const = 0;
+  virtual gfx::Size GetNinePatchCanvasSize(Part part) const = 0;
+  virtual gfx::Rect GetNinePatchAperture(Part part) const = 0;
 
   // Supports theme specific colors.
   void SetScrollbarColors(unsigned inactive_color,