Add explicit keyword to constructors that take one argument

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 48 matching lines @@
 
     bool hasFailedOrCanceledSubresources() const;
 
     PassRefPtr<CSSImageSetValue> cloneForCSSOM() const;
 
 protected:
     ImageWithScale bestImageForScaleFactor();
 
 private:
     CSSImageSetValue();
-    CSSImageSetValue(const CSSImageSetValue& cloneFrom);
+    explicit CSSImageSetValue(const CSSImageSetValue& cloneFrom);
 
     void fillImageSet();
     static inline bool compareByScaleFactor(ImageWithScale first, ImageWithScale second) { return first.scaleFactor < second.scaleFactor; }
 
     RefPtr<StyleImage> m_imageSet;
     bool m_accessedBestFitImage;
 
     // This represents the scale factor that we used to find the best fit image. It does not necessarily
     // correspond to the scale factor of the best fit image.
     float m_scaleFactor;
 
     Vector<ImageWithScale> m_imagesInSet;
 };
 
 } // namespace WebCore
 
 #endif // CSSImageSetValue_h