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

content/child/webthemeengine_impl_default.h

Index: content/child/webthemeengine_impl_default.h
diff --git a/content/child/webthemeengine_impl_default.h b/content/child/webthemeengine_impl_default.h
index 6f72953fc82f5dc1094750a37ce533efb5bd0865..02d2337e0b2377fb3fd430de9bc29d1c84a9b7d3 100644
--- a/content/child/webthemeengine_impl_default.h
+++ b/content/child/webthemeengine_impl_default.h
@@ -23,6 +23,9 @@ class WebThemeEngineImpl : public blink::WebThemeEngine {
              const blink::WebThemeEngine::ExtraParams* extra_params) override;
   void getOverlayScrollbarStyle(
       blink::WebThemeEngine::ScrollbarStyle*) override;
+  bool supportsNinePatch(Part part) const override;
+  blink::WebSize ninePatchCanvasSize(Part part) const override;
+  blink::WebRect ninePatchAperture(Part part) const override;
 #if defined(OS_WIN)
   // Caches the scrollbar metrics. These are retrieved in the browser and passed
   // to the renderer in RendererPreferences because the required Windows system