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

content/child/webthemeengine_impl_default.h

 // Copyright 2014 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 CONTENT_CHILD_WEBTHEMEENGINE_IMPL_DEFAULT_H_
 #define CONTENT_CHILD_WEBTHEMEENGINE_IMPL_DEFAULT_H_
 
 #include <stdint.h>
 
 #include "build/build_config.h"
 #include "third_party/WebKit/public/platform/WebThemeEngine.h"
 
 namespace content {
 
 class WebThemeEngineImpl : public blink::WebThemeEngine {
  public:
   // WebThemeEngine methods:
   blink::WebSize getSize(blink::WebThemeEngine::Part) override;
   void paint(blink::WebCanvas* canvas,
              blink::WebThemeEngine::Part part,
              blink::WebThemeEngine::State state,
              const blink::WebRect& rect,
              const blink::WebThemeEngine::ExtraParams* extra_params) override;
   void getOverlayScrollbarStyle(
       blink::WebThemeEngine::ScrollbarStyle*) override;
+  bool supportsNinePatch(Part part) const override;
+  blink::WebSize ninePatchCanvasSize(Part part) const override;
+  blink::WebRect ninePatchAperture(Part part) const override;
 #if defined(OS_WIN)
   // Caches the scrollbar metrics. These are retrieved in the browser and passed
   // to the renderer in RendererPreferences because the required Windows system
   // calls cannot be made in sandboxed renderers.
   static void cacheScrollBarMetrics(int32_t vertical_scroll_bar_width,
                                     int32_t horizontal_scroll_bar_height,
                                     int32_t vertical_arrow_bitmap_height,
                                     int32_t horizontal_arrow_bitmap_width);
 #endif
 };
 
 }  // namespace content
 
 #endif  // CONTENT_CHILD_WEBTHEMEENGINE_IMPL_DEFAULT_H_