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

third_party/WebKit/Source/platform/testing/TestingPlatformSupport.cpp

Index: third_party/WebKit/Source/platform/testing/TestingPlatformSupport.cpp
diff --git a/third_party/WebKit/Source/platform/testing/TestingPlatformSupport.cpp b/third_party/WebKit/Source/platform/testing/TestingPlatformSupport.cpp
index 42251b0e09c04f1f965b6847511014d35819d10c..8c4e5cf9ed6e9990501194bb747946e2667e5f05 100644
--- a/third_party/WebKit/Source/platform/testing/TestingPlatformSupport.cpp
+++ b/third_party/WebKit/Source/platform/testing/TestingPlatformSupport.cpp
@@ -123,6 +123,14 @@ TestingCompositorSupport::createScrollbarLayer(
 }
 
 std::unique_ptr<WebScrollbarLayer>
+TestingCompositorSupport::createOverlayScrollbarLayer(
+    std::unique_ptr<WebScrollbar>,
+    WebScrollbarThemePainter,
+    std::unique_ptr<WebScrollbarThemeGeometry>) {
+  return nullptr;
+}
+
+std::unique_ptr<WebScrollbarLayer>
 TestingCompositorSupport::createSolidColorScrollbarLayer(
     WebScrollbar::Orientation,
     int thumbThickness,