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

third_party/WebKit/Source/core/style/ComputedStyle.cpp

 /*
  * Copyright (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights
  * reserved.
  * Copyright (C) 2011 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 903 matching lines @@
   bool other_has_clip =
       other.HasOutOfFlowPosition() && !other.visual_data_->has_auto_clip_;
   if (has_clip != other_has_clip ||
       (has_clip && visual_data_->clip_ != other.visual_data_->clip_))
     diff.SetCSSClipChanged();
 }
 
 void ComputedStyle::AddPaintImage(StyleImage* image) {
   if (!rare_non_inherited_data_.Access()->paint_images_) {
     rare_non_inherited_data_.Access()->paint_images_ =
-        WTF::MakeUnique<Vector<Persistent<StyleImage>>>();
+        WTF::MakeUnique<PaintImages>();
   }
   rare_non_inherited_data_.Access()->paint_images_->push_back(image);
 }
 
 void ComputedStyle::AddCursor(StyleImage* image,
                               bool hot_spot_specified,
                               const IntPoint& hot_spot) {
   if (!rare_inherited_data_.Access()->cursor_data_)
     rare_inherited_data_.Access()->cursor_data_ = new CursorList;
   rare_inherited_data_.Access()->cursor_data_->push_back(
@@ skipping 1437 matching lines @@
     if (value < 0)
       fvalue -= 0.5f;
     else
       fvalue += 0.5f;
   }
 
   return RoundForImpreciseConversion<int>(fvalue / zoom_factor);
 }
 
 }  // namespace blink