Remove 'String::append' from some of the blink source.

Source/core/fetch/XSLStyleSheetResource.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, 2007, 2008 Apple Inc. All rights reserved.
 
     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 50 matching lines @@
     m_decoder->setEncoding(chs, TextResourceDecoder::EncodingFromHTTPHeader);
 }
 
 String XSLStyleSheetResource::encoding() const
 {
     return m_decoder->encoding().name();
 }
 
 void XSLStyleSheetResource::checkNotify()
 {
-    if (m_data.get()) {
-        m_sheet = m_decoder->decode(m_data->data(), encodedSize());
-        m_sheet.append(m_decoder->flush());
-    }
+    if (m_data.get())
+        m_sheet = m_decoder->decode(m_data->data(), encodedSize()) + m_decoder->flush();
 
     ResourceClientWalker<StyleSheetResourceClient> w(m_clients);
     while (StyleSheetResourceClient* c = w.next())
         c->setXSLStyleSheet(m_resourceRequest.url(), m_response.url(), m_sheet);
 }
 
 } // namespace WebCore