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

third_party/WebKit/public/platform/WebCompositorSupport.h

Index: third_party/WebKit/public/platform/WebCompositorSupport.h
diff --git a/third_party/WebKit/public/platform/WebCompositorSupport.h b/third_party/WebKit/public/platform/WebCompositorSupport.h
index 0ef1b3cae2ef242aabb671c3339e263b01e70d45..5c3f706aaa1df1a1fad42dc185e13156fe8d99bd 100644
--- a/third_party/WebKit/public/platform/WebCompositorSupport.h
+++ b/third_party/WebKit/public/platform/WebCompositorSupport.h
@@ -70,6 +70,11 @@ class WebCompositorSupport {
       WebScrollbarThemePainter,
       std::unique_ptr<WebScrollbarThemeGeometry>) = 0;
 
+  virtual std::unique_ptr<WebScrollbarLayer> createOverlayScrollbarLayer(
+      std::unique_ptr<WebScrollbar>,
+      WebScrollbarThemePainter,
+      std::unique_ptr<WebScrollbarThemeGeometry>) = 0;
+
   virtual std::unique_ptr<WebScrollbarLayer> createSolidColorScrollbarLayer(
       WebScrollbar::Orientation,
       int thumbThickness,