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

cc/blink/web_compositor_support_impl.h

Index: cc/blink/web_compositor_support_impl.h
diff --git a/cc/blink/web_compositor_support_impl.h b/cc/blink/web_compositor_support_impl.h
index 91a205a12a300525d05bef3146dfc8c19a8e2561..a303b273122f181ba943af090b5aff455fc24aa8 100644
--- a/cc/blink/web_compositor_support_impl.h
+++ b/cc/blink/web_compositor_support_impl.h
@@ -31,6 +31,10 @@ class CC_BLINK_EXPORT WebCompositorSupportImpl
       std::unique_ptr<blink::WebScrollbar> scrollbar,
       blink::WebScrollbarThemePainter painter,
       std::unique_ptr<blink::WebScrollbarThemeGeometry>) override;
+  std::unique_ptr<blink::WebScrollbarLayer> createOverlayScrollbarLayer(
+      std::unique_ptr<blink::WebScrollbar> scrollbar,
+      blink::WebScrollbarThemePainter painter,
+      std::unique_ptr<blink::WebScrollbarThemeGeometry>) override;
   std::unique_ptr<blink::WebScrollbarLayer> createSolidColorScrollbarLayer(
       blink::WebScrollbar::Orientation orientation,
       int thumb_thickness,