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

cc/blink/scrollbar_impl.h

Index: cc/blink/scrollbar_impl.h
diff --git a/cc/blink/scrollbar_impl.h b/cc/blink/scrollbar_impl.h
index d5c3052c40728f0a909c6e5a047226d4818a3e94..c0534d5b5e7f6ad4f3b6b93fb40d12a53b195a0e 100644
--- a/cc/blink/scrollbar_impl.h
+++ b/cc/blink/scrollbar_impl.h
@@ -41,6 +41,10 @@ class ScrollbarImpl : public cc::Scrollbar {
                  cc::ScrollbarPart part,
                  const gfx::Rect& content_rect) override;
 
+  bool UsesNinePatchThumbResource() const override;
+  gfx::Size NinePatchThumbCanvasSize() const override;
+  gfx::Rect NinePatchThumbAperture() const override;
+
  private:
   std::unique_ptr<blink::WebScrollbar> scrollbar_;
   blink::WebScrollbarThemePainter painter_;