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

cc/blink/web_compositor_support_impl.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 CC_BLINK_WEB_COMPOSITOR_SUPPORT_IMPL_H_
 #define CC_BLINK_WEB_COMPOSITOR_SUPPORT_IMPL_H_
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "cc/blink/cc_blink_export.h"
@@ skipping 13 matching lines @@
   std::unique_ptr<blink::WebLayer> createLayerFromCCLayer(cc::Layer*) override;
   std::unique_ptr<blink::WebContentLayer> createContentLayer(
       blink::WebContentLayerClient* client) override;
   std::unique_ptr<blink::WebExternalTextureLayer> createExternalTextureLayer(
       cc::TextureLayerClient* client) override;
   std::unique_ptr<blink::WebImageLayer> createImageLayer() override;
   std::unique_ptr<blink::WebScrollbarLayer> createScrollbarLayer(
       std::unique_ptr<blink::WebScrollbar> scrollbar,
       blink::WebScrollbarThemePainter painter,
       std::unique_ptr<blink::WebScrollbarThemeGeometry>) override;
+  std::unique_ptr<blink::WebScrollbarLayer> createOverlayScrollbarLayer(
+      std::unique_ptr<blink::WebScrollbar> scrollbar,
+      blink::WebScrollbarThemePainter painter,
+      std::unique_ptr<blink::WebScrollbarThemeGeometry>) override;
   std::unique_ptr<blink::WebScrollbarLayer> createSolidColorScrollbarLayer(
       blink::WebScrollbar::Orientation orientation,
       int thumb_thickness,
       int track_start,
       bool is_left_side_vertical_scrollbar) override;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(WebCompositorSupportImpl);
 };
 
 }  // namespace cc_blink
 
 #endif  // CC_BLINK_WEB_COMPOSITOR_SUPPORT_IMPL_H_