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

third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 327 matching lines @@
 }
 
 static std::unique_ptr<WebScrollbarLayer> createScrollbarLayer(
     Scrollbar& scrollbar,
     float deviceScaleFactor) {
   ScrollbarTheme& theme = scrollbar.theme();
   WebScrollbarThemePainter painter(theme, scrollbar, deviceScaleFactor);
   std::unique_ptr<WebScrollbarThemeGeometry> geometry(
       WebScrollbarThemeGeometryNative::create(theme));
 
-  std::unique_ptr<WebScrollbarLayer> scrollbarLayer =
-      Platform::current()->compositorSupport()->createScrollbarLayer(
-          WebScrollbarImpl::create(&scrollbar), painter, std::move(geometry));
+  std::unique_ptr<WebScrollbarLayer> scrollbarLayer;
+  if (theme.usesOverlayScrollbars() && theme.usesNinePatchThumbResource()) {
+    scrollbarLayer =
+        Platform::current()->compositorSupport()->createOverlayScrollbarLayer(
+            WebScrollbarImpl::create(&scrollbar), painter, std::move(geometry));
+  } else {
+    scrollbarLayer =
+        Platform::current()->compositorSupport()->createScrollbarLayer(
+            WebScrollbarImpl::create(&scrollbar), painter, std::move(geometry));
+  }
   GraphicsLayer::registerContentsLayer(scrollbarLayer->layer());
   return scrollbarLayer;
 }
 
 std::unique_ptr<WebScrollbarLayer>
 ScrollingCoordinator::createSolidColorScrollbarLayer(
     ScrollbarOrientation orientation,
     int thumbThickness,
     int trackStart,
     bool isLeftSideVerticalScrollbar) {
@@ skipping 827 matching lines @@
           frameView ? toWebLayer(frameView->layoutViewportScrollableArea()
                                      ->layerForScrolling())
                     : nullptr) {
     return WebSize(frameView->layoutViewportScrollableArea()->contentsSize()) !=
            scrollLayer->bounds();
   }
   return false;
 }
 
 }  // namespace blink