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

ui/native_theme/native_theme_base.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_BASE_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_BASE_H_
 
 #include <memory>
 
 #include "base/compiler_specific.h"
@@ skipping 15 matching lines @@
   // NativeTheme implementation:
   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;
 
+  bool SupportsNinePatch(Part part) const override;
+  gfx::Size GetNinePatchCanvasSize(Part part) const override;
+  gfx::Rect GetNinePatchAperture(Part part) const override;
+
  protected:
   NativeThemeBase();
   ~NativeThemeBase() override;
 
   // Draw the arrow. Used by scrollbar and inner spin button.
   virtual void PaintArrowButton(
       SkCanvas* gc,
       const gfx::Rect& rect,
       Part direction,
       State state) const;
@@ skipping 143 matching lines @@
 
   // The length of the arrow buttons, 0 means no buttons are drawn.
   int scrollbar_button_length_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeThemeBase);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_BASE_H_