Have HashMap<KeyType, AtomicString>::get() return a const reference

Source/core/xml/XMLHttpRequest.h

 /*
  *  Copyright (C) 2003, 2006, 2008 Apple Inc. All rights reserved.
  *  Copyright (C) 2005, 2006 Alexey Proskuryakov <ap@nypop.com>
  *  Copyright (C) 2011 Google Inc. All rights reserved.
  *  Copyright (C) 2012 Intel Corporation
  *
  *  This library is free software; you can redistribute it and/or
  *  modify it under the terms of the GNU Lesser General Public
  *  License as published by the Free Software Foundation; either
  *  version 2 of the License, or (at your option) any later version.
@@ skipping 95 matching lines @@
     void send(Document*, ExceptionState&);
     void send(const String&, ExceptionState&);
     void send(Blob*, ExceptionState&);
     void send(DOMFormData*, ExceptionState&);
     void send(ArrayBuffer*, ExceptionState&);
     void send(ArrayBufferView*, ExceptionState&);
     void abort();
     void setRequestHeader(const AtomicString& name, const AtomicString& value, ExceptionState&);
     void overrideMimeType(const AtomicString& override);
     String getAllResponseHeaders(ExceptionState&) const;
-    AtomicString getResponseHeader(const AtomicString& name, ExceptionState&) const;
+    const AtomicString& getResponseHeader(const AtomicString& name, ExceptionState&) const;
     ScriptString responseText(ExceptionState&);
     ScriptString responseJSONSource();
     Document* responseXML(ExceptionState&);
     Blob* responseBlob();
     Stream* responseStream();
     unsigned long timeout() const { return m_timeoutMilliseconds; }
     void setTimeout(unsigned long timeout, ExceptionState&);
 
     void sendForInspectorXHRReplay(PassRefPtr<FormData>, ExceptionState&);
 
@@ skipping 33 matching lines @@
     virtual void didFailRedirectCheck();
 
     AtomicString responseMIMEType() const;
     bool responseIsXML() const;
 
     bool areMethodAndURLValidForSend();
 
     bool initSend(ExceptionState&);
     void sendBytesData(const void*, size_t, ExceptionState&);
 
-    AtomicString getRequestHeader(const AtomicString& name) const;
+    const AtomicString& getRequestHeader(const AtomicString& name) const;
     void setRequestHeaderInternal(const AtomicString& name, const AtomicString& value);
 
     void trackProgress(int dataLength);
     // Changes m_state and dispatches a readyStateChange event if new m_state
     // value is different from last one.
     void changeState(State newState);
     void dispatchReadyStateChangeEvent();
 
     void dropProtectionSoon();
     void dropProtection();
@@ skipping 80 matching lines @@
 
     // An enum corresponding to the allowed string values for the responseType attribute.
     ResponseTypeCode m_responseTypeCode;
     AsyncMethodRunner<XMLHttpRequest> m_dropProtectionRunner;
     RefPtr<SecurityOrigin> m_securityOrigin;
 };
 
 } // namespace WebCore
 
 #endif // XMLHttpRequest_h