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..7aa469a50b537f2d293d79f5cbd92bfd7f5c76f8 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 +269,20 @@ class NATIVE_THEME_EXPORT NativeTheme {
                                     const gfx::Rect& rect,
                                     ScrollbarOverlayColorTheme theme) const {}
 
+  // Returns whether the theme uses a nine-patch resource for the given part.
+  // If true, calling code should always paint into a canvas the size of which
+  // can be gotten from GetNinePatchCanvasSize.
+  virtual bool SupportsNinePatch(Part part) const = 0;
+
+  // If the part paints into a nine-patch resource, the size of the canvas
+  // which should be painted into.
+  virtual gfx::Size GetNinePatchCanvasSize(Part part) const = 0;
+
+  // If the part paints into a nine-patch resource, the rect in the canvas
+  // which defines the center tile. This is the tile that should be resized out
+  // when the part is resized.
+  virtual gfx::Rect GetNinePatchAperture(Part part) const = 0;
+
   // Supports theme specific colors.
   void SetScrollbarColors(unsigned inactive_color,
                           unsigned active_color,