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

third_party/WebKit/Source/platform/testing/TestingPlatformSupport.h

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 59 matching lines @@
   std::unique_ptr<WebLayer> createLayerFromCCLayer(cc::Layer*) override;
   std::unique_ptr<WebContentLayer> createContentLayer(
       WebContentLayerClient*) override;
   std::unique_ptr<WebExternalTextureLayer> createExternalTextureLayer(
       cc::TextureLayerClient*) override;
   std::unique_ptr<WebImageLayer> createImageLayer() override;
   std::unique_ptr<WebScrollbarLayer> createScrollbarLayer(
       std::unique_ptr<WebScrollbar>,
       WebScrollbarThemePainter,
       std::unique_ptr<WebScrollbarThemeGeometry>) override;
+  std::unique_ptr<WebScrollbarLayer> createOverlayScrollbarLayer(
+      std::unique_ptr<WebScrollbar>,
+      WebScrollbarThemePainter,
+      std::unique_ptr<WebScrollbarThemeGeometry>) override;
   std::unique_ptr<WebScrollbarLayer> createSolidColorScrollbarLayer(
       WebScrollbar::Orientation,
       int thumbThickness,
       int trackStart,
       bool isLeftSideVerticalScrollbar) override;
 };
 
 // A base class to override Platform methods for testing.  You can override the
 // behavior by subclassing TestingPlatformSupport or using
 // ScopedTestingPlatformSupport (see below).
@@ skipping 142 matching lines @@
       m_discardableMemoryAllocator;
   std::unique_ptr<DummyPlatform> m_dummyPlatform;
   std::unique_ptr<cc_blink::WebCompositorSupportImpl> m_compositorSupport;
   TestingPlatformSupport::Config m_testingPlatformConfig;
   std::unique_ptr<TestingPlatformSupport> m_testingPlatformSupport;
 };
 
 }  // namespace blink
 
 #endif  // TestingPlatformSupport_h