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

ui/native_theme/native_theme_win.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_NATIVE_THEME_NATIVE_THEME_WIN_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_WIN_H_
 
 // A wrapper class for working with custom XP/Vista themes provided in
 // uxtheme.dll.  This is a singleton class that can be grabbed using
 // NativeThemeWin::instance().
@@ skipping 91 matching lines @@
   gfx::Size GetPartSize(Part part,
                         State state,
                         const ExtraParams& extra) const override;
   void Paint(SkCanvas* canvas,
              Part part,
              State state,
              const gfx::Rect& rect,
              const ExtraParams& extra) const override;
   SkColor GetSystemColor(ColorId color_id) const override;
 
+  bool SupportsNinePatch(Part part) const override;
+  gfx::Size GetNinePatchCanvasSize(Part part) const override;
+  gfx::Rect GetNinePatchAperture(Part part) const override;
+
  protected:
   friend class NativeTheme;
   // Gets our singleton instance.
   static NativeThemeWin* instance();
 
   NativeThemeWin();
   ~NativeThemeWin() override;
 
  private:
   bool IsUsingHighContrastThemeInternal();
@@ skipping 250 matching lines @@
 
   // Is |is_using_high_contrast_| valid?
   mutable bool is_using_high_contrast_valid_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeThemeWin);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_WIN_H_