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

ui/views/view_unittest.cc

Index: ui/views/view_unittest.cc
diff --git a/ui/views/view_unittest.cc b/ui/views/view_unittest.cc
index 4753be7534ecfdafab19aeca838dc10be686456c..6fe827757635ba4d434f6fdc27c04c5466233d98 100644
--- a/ui/views/view_unittest.cc
+++ b/ui/views/view_unittest.cc
@@ -4565,6 +4565,14 @@ class TestNativeTheme : public ui::NativeTheme {
              const gfx::Rect& rect,
              const ExtraParams& extra) const override {}
 
+  bool SupportsNinePatch(Part part) const override { return false; }
+  gfx::Size GetNinePatchCanvasSize(Part part) const override {
+    return gfx::Size();
+  }
+  gfx::Rect GetNinePatchAperture(Part part) const override {
+    return gfx::Rect();
+  }
+
  private:
   DISALLOW_COPY_AND_ASSIGN(TestNativeTheme);
 };