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

Source/core/fetch/ResourceFetcher.cpp

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller (mueller@kde.org)
     Copyright (C) 2002 Waldo Bastian (bastian@kde.org)
     Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
     Copyright (C) 2009 Torch Mobile Inc. http://www.torchmobile.com/
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
@@ skipping 724 matching lines @@
 {
     ASSERT(resource);
     ASSERT(resource->inCache());
     ASSERT(resource->isLoaded());
     ASSERT(resource->canUseCacheValidator());
     ASSERT(!resource->resourceToRevalidate());
 
     ResourceRequest revalidatingRequest(resource->resourceRequest());
     addAdditionalRequestHeaders(revalidatingRequest, resource->type());
 
-    const String& lastModified = resource->response().httpHeaderField("Last-Modified");
-    const String& eTag = resource->response().httpHeaderField("ETag");
+    const AtomicString& lastModified = resource->response().httpHeaderField("Last-Modified");
+    const AtomicString& eTag = resource->response().httpHeaderField("ETag");
     if (!lastModified.isEmpty() || !eTag.isEmpty()) {
         ASSERT(context().cachePolicy(resource->type()) != CachePolicyReload);
         if (context().cachePolicy(resource->type()) == CachePolicyRevalidate)
             revalidatingRequest.setHTTPHeaderField("Cache-Control", "max-age=0");
         if (!lastModified.isEmpty())
             revalidatingRequest.setHTTPHeaderField("If-Modified-Since", lastModified);
         if (!eTag.isEmpty())
             revalidatingRequest.setHTTPHeaderField("If-None-Match", eTag);
     }
 
@@ skipping 576 matching lines @@
 }
 #endif
 
 const ResourceLoaderOptions& ResourceFetcher::defaultResourceOptions()
 {
     DEFINE_STATIC_LOCAL(ResourceLoaderOptions, options, (SendCallbacks, SniffContent, BufferData, AllowStoredCredentials, ClientRequestedCredentials, AskClientForCrossOriginCredentials, DoSecurityCheck, CheckContentSecurityPolicy, DocumentContext));
     return options;
 }
 
 }