BackgroundImage incorrectly returns empty url() when created on-the-fly

Source/core/style/StyleFetchedImageSet.h

 /*
  * Copyright (C) 2012 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 30 matching lines @@
 class StyleFetchedImageSet final : public StyleImage, private ImageResourceClient {
     WTF_MAKE_FAST_ALLOCATED_WILL_BE_REMOVED(StyleFetchedImageSet);
 public:
     static PassRefPtrWillBeRawPtr<StyleFetchedImageSet> create(ImageResource* image, float imageScaleFactor, CSSImageSetValue* value)
     {
         return adoptRefWillBeNoop(new StyleFetchedImageSet(image, imageScaleFactor, value));
     }
     ~StyleFetchedImageSet() override;
 
     PassRefPtrWillBeRawPtr<CSSValue> cssValue() const override;
+    PassRefPtrWillBeRawPtr<CSSValue> computedCSSValue() const override;
 
     // FIXME: This is used by StyleImage for equals comparison, but this implementation
     // only looks at the image from the set that we have loaded. I'm not sure if that is
     // meaningful enough or not.
     WrappedImagePtr data() const override { return m_bestFitImage.get(); }
 
 #if !ENABLE(OILPAN)
     void clearImageSetValue() { m_imageSetValue = nullptr; }
 #endif
 
@@ skipping 22 matching lines @@
     float m_imageScaleFactor;
 
     RawPtrWillBeMember<CSSImageSetValue> m_imageSetValue; // Not retained; it owns us.
 };
 
 DEFINE_STYLE_IMAGE_TYPE_CASTS(StyleFetchedImageSet, isImageResourceSet());
 
 } // namespace blink
 
 #endif // StyleFetchedImageSet_h