One more instance of StringBuilder::Append replacing String::Append.

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 80 matching lines @@
     String sheetText = m_decoder->decode(m_data->data(), m_data->size());
     sheetText = sheetText + m_decoder->flush();
     return sheetText;
 }
 
 void CSSStyleSheetResource::checkNotify()
 {
     // Decode the data to find out the encoding and keep the sheet text around during checkNotify()
     if (m_data) {
         m_decodedSheetText = m_decoder->decode(m_data->data(), m_data->size());
-        m_decodedSheetText.append(m_decoder->flush());
+        m_decodedSheetText = m_decodedSheetText + m_decoder->flush();
     }
 
     ResourceClientWalker<StyleSheetResourceClient> w(m_clients);
     while (StyleSheetResourceClient* c = w.next())
         c->setCSSStyleSheet(m_resourceRequest.url(), m_response.url(), m_decoder->encoding().name(), this);
     // Clear the decoded text as it is unlikely to be needed immediately again and is cheap to regenerate.
     m_decodedSheetText = String();
 }
 
 bool CSSStyleSheetResource::canUseSheet(bool enforceMIMEType, bool* hasValidMIMEType) const
@@ skipping 62 matching lines @@
 
     if (m_parsedStyleSheetCache)
         m_parsedStyleSheetCache->removedFromMemoryCache();
     m_parsedStyleSheetCache = sheet;
     m_parsedStyleSheetCache->addedToMemoryCache();
 
     setDecodedSize(m_parsedStyleSheetCache->estimatedSizeInBytes());
 }
 
 }