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

cc/blink/web_compositor_support_impl.cc

Index: cc/blink/web_compositor_support_impl.cc
diff --git a/cc/blink/web_compositor_support_impl.cc b/cc/blink/web_compositor_support_impl.cc
index ac2b81589d687592b6428e2e44c7a20122be4eec..7bb783e571639f56cf8bce033cc2e443cd95b58d 100644
--- a/cc/blink/web_compositor_support_impl.cc
+++ b/cc/blink/web_compositor_support_impl.cc
@@ -71,6 +71,15 @@ WebCompositorSupportImpl::createScrollbarLayer(
 }
 
 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,