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 b9a7b6969a3e8f258d89cf65f7c49e9f7e765353..00961927f03e7e98f549a25a590cc7efc12ffa46 100644
--- a/ui/native_theme/native_theme.h
+++ b/ui/native_theme/native_theme.h
@@ -11,6 +11,8 @@
 #include "cc/paint/paint_canvas.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "ui/base/models/menu_separator_types.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"
 
@@ -267,6 +269,10 @@ class NATIVE_THEME_EXPORT NativeTheme {
                                     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,
                           unsigned active_color,