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 4e4bdaedc25e7a5e2e304a71280cb06c61b7ebd1..541b4b32f79ee8de9e58083f3772c3180d6a22ba 100644
--- a/cc/blink/scrollbar_impl.h
+++ b/cc/blink/scrollbar_impl.h
@@ -40,6 +40,10 @@ 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_;