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

Source/core/fetch/CSSStyleSheetResource.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) 2006 Samuel Weinig (sam.weinig@gmail.com)
     Copyright (C) 2004, 2005, 2006 Apple Computer, Inc.
 
     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 105 matching lines @@
     if (!enforceMIMEType && !hasValidMIMEType)
         return true;
 
     // This check exactly matches Firefox. Note that we grab the Content-Type
     // header directly because we want to see what the value is BEFORE content
     // sniffing. Firefox does this by setting a "type hint" on the channel.
     // This implementation should be observationally equivalent.
     //
     // This code defaults to allowing the stylesheet for non-HTTP protocols so
     // folks can use standards mode for local HTML documents.
-    AtomicString mimeType = extractMIMETypeFromMediaType(response().httpHeaderField("Content-Type"));
+    const AtomicString& mimeType = extractMIMETypeFromMediaType(response().httpHeaderField("Content-Type"));
     bool typeOK = mimeType.isEmpty() || equalIgnoringCase(mimeType, "text/css") || equalIgnoringCase(mimeType, "application/x-unknown-content-type");
     if (hasValidMIMEType)
         *hasValidMIMEType = typeOK;
     if (!enforceMIMEType)
         return true;
     return typeOK;
 }
 
 void CSSStyleSheetResource::destroyDecodedData()
 {
@@ skipping 37 matching lines @@
 
     if (m_parsedStyleSheetCache)
         m_parsedStyleSheetCache->removedFromMemoryCache();
     m_parsedStyleSheetCache = sheet;
     m_parsedStyleSheetCache->addedToMemoryCache();
 
     setDecodedSize(m_parsedStyleSheetCache->estimatedSizeInBytes());
 }
 
 }