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

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

 /*
  * 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 107 matching lines @@
 
 std::unique_ptr<WebScrollbarLayer>
 TestingCompositorSupport::createScrollbarLayer(
     std::unique_ptr<WebScrollbar>,
     WebScrollbarThemePainter,
     std::unique_ptr<WebScrollbarThemeGeometry>) {
   return nullptr;
 }
 
 std::unique_ptr<WebScrollbarLayer>
+TestingCompositorSupport::createOverlayScrollbarLayer(
+    std::unique_ptr<WebScrollbar>,
+    WebScrollbarThemePainter,
+    std::unique_ptr<WebScrollbarThemeGeometry>) {
+  return nullptr;
+}
+
+std::unique_ptr<WebScrollbarLayer>
 TestingCompositorSupport::createSolidColorScrollbarLayer(
     WebScrollbar::Orientation,
     int thumbThickness,
     int trackStart,
     bool isLeftSideVerticalScrollbar) {
   return nullptr;
 }
 
 TestingPlatformSupport::TestingPlatformSupport()
     : TestingPlatformSupport(TestingPlatformSupport::Config()) {}
@@ skipping 204 matching lines @@
   ThreadState::attachMainThread();
   ThreadState::current()->registerTraceDOMWrappers(nullptr, nullptr, nullptr,
                                                    nullptr);
   HTTPNames::init();
   FetchInitiatorTypeNames::init();
 }
 
 ScopedUnittestsEnvironmentSetup::~ScopedUnittestsEnvironmentSetup() {}
 
 }  // namespace blink