Update HTTPHeaderMap wrappers to use AtomicString type for header values

Source/platform/network/ResourceResponse.h

 /*
  * Copyright (C) 2006, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google 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
@@ skipping 67 matching lines @@
     // There should not be a setter exposed, as suggested file name is determined based on other headers in a manner that WebCore does not necessarily know about.
     const String& suggestedFilename() const;
     void setSuggestedFilename(const String&);
 
     int httpStatusCode() const;
     void setHTTPStatusCode(int);
 
     const AtomicString& httpStatusText() const;
     void setHTTPStatusText(const AtomicString&);
 
-    String httpHeaderField(const AtomicString& name) const;
-    String httpHeaderField(const char* name) const;
-    void setHTTPHeaderField(const AtomicString& name, const String& value);
-    void addHTTPHeaderField(const AtomicString& name, const String& value);
+    AtomicString httpHeaderField(const AtomicString& name) const;
+    const AtomicString& httpHeaderField(const char* name) const;
+    void setHTTPHeaderField(const AtomicString& name, const AtomicString& value);
+    void addHTTPHeaderField(const AtomicString& name, const AtomicString& value);
     void clearHTTPHeaderField(const AtomicString& name);
     const HTTPHeaderMap& httpHeaderFields() const;
 
     bool isMultipart() const { return mimeType() == "multipart/x-mixed-replace"; }
 
     bool isAttachment() const;
 
     // FIXME: These are used by PluginStream on some platforms. Calculations may differ from just returning plain Last-Modified header.
     // Leaving it for now but this should go away in favor of generic solution.
     void setLastModifiedDate(time_t);
@@ skipping 203 matching lines @@
     double m_responseTime;
     String m_remoteIPAddress;
     unsigned short m_remotePort;
     String m_downloadedFilePath;
     RefPtr<BlobDataHandle> m_downloadedFileHandle;
 };
 
 } // namespace WebCore
 
 #endif // ResourceResponse_h