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 15 matching lines @@
 
 #include "config.h"
 #include "core/fetch/XSLStyleSheetResource.h"
 
 #include "RuntimeEnabledFeatures.h"
 #include "core/fetch/ResourceClientWalker.h"
 #include "core/fetch/StyleSheetResourceClient.h"
 #include "core/fetch/TextResourceDecoder.h"
 #include "platform/SharedBuffer.h"
 #include "wtf/Vector.h"
+#include "wtf/text/StringBuilder.h"
 
 namespace WebCore {
 
 XSLStyleSheetResource::XSLStyleSheetResource(const ResourceRequest& resourceRequest)
     : StyleSheetResource(resourceRequest, XSLStyleSheet)
     , m_decoder(TextResourceDecoder::create("text/xsl"))
 {
     ASSERT(RuntimeEnabledFeatures::xsltEnabled());
     DEFINE_STATIC_LOCAL(const AtomicString, acceptXSLT, ("text/xml, application/xml, application/xhtml+xml, text/xsl, application/rss+xml, application/atom+xml", AtomicString::ConstructFromLiteral));
 
@@ skipping 15 matching lines @@
     m_decoder->setEncoding(chs, TextResourceDecoder::EncodingFromHTTPHeader);
 }
 
 String XSLStyleSheetResource::encoding() const
 {
     return m_decoder->encoding().name();
 }
 
 void XSLStyleSheetResource::checkNotify()
 {
+    StringBuilder sheet;
     if (m_data.get()) {
-        m_sheet = m_decoder->decode(m_data->data(), encodedSize());
-        m_sheet.append(m_decoder->flush());
+        sheet.append(m_decoder->decode(m_data->data(), encodedSize()));
+        sheet.append(m_decoder->flush());
+        m_sheet = sheet.toString();
     }
 
     ResourceClientWalker<StyleSheetResourceClient> w(m_clients);
     while (StyleSheetResourceClient* c = w.next())
         c->setXSLStyleSheet(m_resourceRequest.url(), m_response.url(), m_sheet);
 }
 
 } // namespace WebCore