Make PaintImages a class and implement a Clone() method.

third_party/WebKit/Source/core/style/PaintImages.h

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef PaintImages_h
+#define PaintImages_h
+
+#include <memory>
+#include "core/style/StyleImage.h"
+#include "platform/heap/Persistent.h"
+#include "platform/wtf/PtrUtil.h"
+#include "platform/wtf/Vector.h"
+
+namespace blink {
+
+// Not to be deleted through a pointer to the base class.
+class PaintImages : public Vector<Persistent<StyleImage>> {
+ public:
+  std::unique_ptr<PaintImages> Clone() const {
+    return WTF::WrapUnique(new PaintImages(*this));
+  }
+};
+
+}  // namespace blink
+
+#endif  // PaintImages_h