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 9ef7e2263e40a7ac2932841d49fc025b5082814f..9ca0d2d09a0b3354409a8748300980d64368fb47 100644
--- a/cc/test/fake_scrollbar.h
+++ b/cc/test/fake_scrollbar.h
@@ -37,6 +37,9 @@ class FakeScrollbar : public Scrollbar {
   void PaintPart(SkCanvas* 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; }