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

third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp b/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
index de68c2a83cc0d2c6f0c380423af48cfda5970320..9b70d9ee8d9070976af25ddfc8021a21c96ed689 100644
--- a/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/third_party/WebKit/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -345,9 +345,16 @@ static std::unique_ptr<WebScrollbarLayer> createScrollbarLayer(
   std::unique_ptr<WebScrollbarThemeGeometry> geometry(
       WebScrollbarThemeGeometryNative::create(theme));
 
-  std::unique_ptr<WebScrollbarLayer> scrollbarLayer =
-      Platform::current()->compositorSupport()->createScrollbarLayer(
-          WebScrollbarImpl::create(&scrollbar), painter, std::move(geometry));
+  std::unique_ptr<WebScrollbarLayer> scrollbarLayer;
+  if (theme.usesOverlayScrollbars() && theme.usesNinePatchThumbResource()) {
+    scrollbarLayer =
+        Platform::current()->compositorSupport()->createOverlayScrollbarLayer(
+            WebScrollbarImpl::create(&scrollbar), painter, std::move(geometry));
+  } else {
+    scrollbarLayer =
+        Platform::current()->compositorSupport()->createScrollbarLayer(
+            WebScrollbarImpl::create(&scrollbar), painter, std::move(geometry));
+  }
   GraphicsLayer::registerContentsLayer(scrollbarLayer->layer());
   return scrollbarLayer;
 }