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

cc/test/fake_scrollbar.h

Index: cc/test/fake_scrollbar.h
diff --git a/cc/test/fake_scrollbar.h b/cc/test/fake_scrollbar.h
index dff1feeec990fb9ec06ec21bddd518a1b51ab1ab..f5a3bd7a10f1570d6098c8b044eb6bd1e0cbb130 100644
--- a/cc/test/fake_scrollbar.h
+++ b/cc/test/fake_scrollbar.h
@@ -37,6 +37,9 @@ class FakeScrollbar : public Scrollbar {
   void PaintPart(PaintCanvas* canvas,
                  ScrollbarPart part,
                  const gfx::Rect& content_rect) override;
+  bool UsesNinePatchThumbResource() const override;
+  gfx::Size NinePatchThumbCanvasSize() const override;
+  gfx::Rect NinePatchThumbAperture() const override;
 
   void set_location(const gfx::Point& location) { location_ = location; }
   void set_track_rect(const gfx::Rect& track_rect) { track_rect_ = track_rect; }