Set proper referrer for css resource fetching

Source/core/css/CSSImageSetValue.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 38 matching lines @@
 
     // Returns a StyleFetchedImageSet if the best fit image has been cached already, otherwise a StylePendingImage.
     StyleImage* cachedOrPendingImageSet(float);
 
     String customCSSText() const;
 
     bool isPending() const { return !m_accessedBestFitImage; }
 
     struct ImageWithScale {
         String imageURL;
+        String referrer;
         float scaleFactor;
     };
 
     bool hasFailedOrCanceledSubresources() const;
 
     PassRefPtrWillBeRawPtr<CSSImageSetValue> cloneForCSSOM() const;
 
     void traceAfterDispatch(Visitor* visitor) { CSSValueList::traceAfterDispatch(visitor); }
 
 protected:
@@ skipping 14 matching lines @@
     float m_scaleFactor;
 
     Vector<ImageWithScale> m_imagesInSet;
 };
 
 DEFINE_CSS_VALUE_TYPE_CASTS(CSSImageSetValue, isImageSetValue());
 
 } // namespace WebCore
 
 #endif // CSSImageSetValue_h