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

cc/blink/web_scrollbar_layer_impl.cc

Index: cc/blink/web_scrollbar_layer_impl.cc
diff --git a/cc/blink/web_scrollbar_layer_impl.cc b/cc/blink/web_scrollbar_layer_impl.cc
index b9c6df38b6c1db6a7e1feb25c181e288e9a02a64..b6a759c51471a71433c6a424f18107ad89a7f368 100644
--- a/cc/blink/web_scrollbar_layer_impl.cc
+++ b/cc/blink/web_scrollbar_layer_impl.cc
@@ -10,10 +10,12 @@
 #include "cc/blink/scrollbar_impl.h"
 #include "cc/blink/web_layer_impl.h"
 #include "cc/layers/layer.h"
+#include "cc/layers/painted_overlay_scrollbar_layer.h"
 #include "cc/layers/painted_scrollbar_layer.h"
 #include "cc/layers/scrollbar_layer_interface.h"
 #include "cc/layers/solid_color_scrollbar_layer.h"
 
+using cc::PaintedOverlayScrollbarLayer;
 using cc::PaintedScrollbarLayer;
 using cc::SolidColorScrollbarLayer;
 
@@ -40,6 +42,17 @@ WebScrollbarLayerImpl::WebScrollbarLayerImpl(
           0))) {}
 
 WebScrollbarLayerImpl::WebScrollbarLayerImpl(
+    std::unique_ptr<blink::WebScrollbar> scrollbar,
+    blink::WebScrollbarThemePainter painter,
+    std::unique_ptr<blink::WebScrollbarThemeGeometry> geometry,
+    bool)
+    : layer_(new WebLayerImpl(PaintedOverlayScrollbarLayer::Create(
+          base::MakeUnique<ScrollbarImpl>(std::move(scrollbar),
+                                          painter,
+                                          std::move(geometry)),
+          0))) {}
+
+WebScrollbarLayerImpl::WebScrollbarLayerImpl(
     blink::WebScrollbar::Orientation orientation,
     int thumb_thickness,
     int track_start,