Change CSSImageValue's member variables from String to AtomicString

Source/core/css/CSSImageValue.cpp

Index: Source/core/css/CSSImageValue.cpp
diff --git a/Source/core/css/CSSImageValue.cpp b/Source/core/css/CSSImageValue.cpp
index c4c6b7908d4f683764658f06c2e0ec6c29f2ac86..909f0824bf1d6c0b1b8cde24b99977f2227ba319 100644
--- a/Source/core/css/CSSImageValue.cpp
+++ b/Source/core/css/CSSImageValue.cpp
@@ -34,7 +34,7 @@
 
 namespace blink {
 
-CSSImageValue::CSSImageValue(const String& rawValue, const KURL& url, StyleImage* image)
+CSSImageValue::CSSImageValue(const AtomicString& rawValue, const KURL& url, StyleImage* image)
     : CSSValue(ImageClass)
     , m_relativeURL(rawValue)
     , m_absoluteURL(url.string())
@@ -125,9 +125,10 @@ DEFINE_TRACE_AFTER_DISPATCH(CSSImageValue)
 void CSSImageValue::reResolveURL(const Document& document)
 {
     KURL url = document.completeURL(m_relativeURL);
-    if (url == m_absoluteURL)
+    AtomicString urlString(url.string());
+    if (urlString == m_absoluteURL)
         return;
-    m_absoluteURL = url.string();
+    m_absoluteURL = urlString;
     m_accessedImage = false;
     m_image.clear();
 }