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

cc/blink/web_compositor_support_impl.cc

 // 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.
 
 #include "cc/blink/web_compositor_support_impl.h"
 
 #include <utility>
 
 #include "base/memory/ptr_util.h"
 #include "cc/blink/web_content_layer_impl.h"
@@ skipping 53 matching lines @@
 std::unique_ptr<WebScrollbarLayer>
 WebCompositorSupportImpl::createScrollbarLayer(
     std::unique_ptr<WebScrollbar> scrollbar,
     WebScrollbarThemePainter painter,
     std::unique_ptr<WebScrollbarThemeGeometry> geometry) {
   return base::MakeUnique<WebScrollbarLayerImpl>(std::move(scrollbar), painter,
                                                  std::move(geometry));
 }
 
 std::unique_ptr<WebScrollbarLayer>
+WebCompositorSupportImpl::createOverlayScrollbarLayer(
+    std::unique_ptr<WebScrollbar> scrollbar,
+    WebScrollbarThemePainter painter,
+    std::unique_ptr<WebScrollbarThemeGeometry> geometry) {
+  return base::MakeUnique<WebScrollbarLayerImpl>(std::move(scrollbar), painter,
+                                                 std::move(geometry), true);
+}
+
+std::unique_ptr<WebScrollbarLayer>
 WebCompositorSupportImpl::createSolidColorScrollbarLayer(
     WebScrollbar::Orientation orientation,
     int thumb_thickness,
     int track_start,
     bool is_left_side_vertical_scrollbar) {
   return base::MakeUnique<WebScrollbarLayerImpl>(
       orientation, thumb_thickness, track_start,
       is_left_side_vertical_scrollbar);
 }
 
 }  // namespace cc_blink