Change CSSImageValue's member variables from String to AtomicString

Source/core/css/CSSImageValue.h

Index: Source/core/css/CSSImageValue.h
diff --git a/Source/core/css/CSSImageValue.h b/Source/core/css/CSSImageValue.h
index 9019164dc9faad43d516699fbb17060e31a8ad31..7bb11887a3b952934e9ba32c103d7626ea15655d 100644
--- a/Source/core/css/CSSImageValue.h
+++ b/Source/core/css/CSSImageValue.h
@@ -38,10 +38,14 @@ class CSSImageValue : public CSSValue {
 public:
     static PassRefPtrWillBeRawPtr<CSSImageValue> create(const KURL& url, StyleImage* image = 0)
     {
-        return adoptRefWillBeNoop(new CSSImageValue(url, url, image));
+        return create(url.string(), url, image);
     }
     static PassRefPtrWillBeRawPtr<CSSImageValue> create(const String& rawValue, const KURL& url, StyleImage* image = 0)
     {
+        return create(AtomicString(rawValue), url, image);
+    }
+    static PassRefPtrWillBeRawPtr<CSSImageValue> create(const AtomicString& rawValue, const KURL& url, StyleImage* image = 0)
+    {
         return adoptRefWillBeNoop(new CSSImageValue(rawValue, url, image));
     }
     ~CSSImageValue();
@@ -72,10 +76,10 @@ public:
     void restoreCachedResourceIfNeeded(Document&);
 
 private:
-    CSSImageValue(const String& rawValue, const KURL&, StyleImage*);
+    CSSImageValue(const AtomicString& rawValue, const KURL&, StyleImage*);
 
-    String m_relativeURL;
-    String m_absoluteURL;
+    AtomicString m_relativeURL;
+    AtomicString m_absoluteURL;
     Referrer m_referrer;
     RefPtrWillBeMember<StyleImage> m_image;
     bool m_accessedImage;