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

Source/platform/network/ResourceRequest.cpp

 /*
  * Copyright (C) 2003, 2006 Apple Computer, Inc.  All rights reserved.
  * Copyright (C) 2009, 2012 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 135 matching lines @@
 void ResourceRequest::setHTTPMethod(const AtomicString& httpMethod)
 {
     m_httpMethod = httpMethod;
 }
 
 const HTTPHeaderMap& ResourceRequest::httpHeaderFields() const
 {
     return m_httpHeaderFields;
 }
 
-AtomicString ResourceRequest::httpHeaderField(const AtomicString& name) const
+const AtomicString& ResourceRequest::httpHeaderField(const AtomicString& name) const
 {
     return m_httpHeaderFields.get(name);
 }
 
 const AtomicString& ResourceRequest::httpHeaderField(const char* name) const
 {
     return m_httpHeaderFields.get(name);
 }
 
 void ResourceRequest::setHTTPHeaderField(const AtomicString& name, const AtomicString& value)
@@ skipping 174 matching lines @@
 // This is used by the loader to control the number of issued parallel load requests.
 unsigned initializeMaximumHTTPConnectionCountPerHost()
 {
     // The chromium network stack already handles limiting the number of
     // parallel requests per host, so there's no need to do it here.  Therefore,
     // this is set to a high value that should never be hit in practice.
     return 10000;
 }
 
 }