Avoid extra copy of CSS data.

Source/core/fetch/TextResource.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "core/fetch/TextResource.h"
 
 #include "core/html/parser/TextResourceDecoder.h"
 #include "platform/SharedBuffer.h"
 
@@ skipping 17 matching lines @@
 String TextResource::encoding() const
 {
     return m_decoder->encoding().name();
 }
 
 String TextResource::decodedText() const
 {
     ASSERT(m_data);
 
     String text = m_decoder->decode(m_data->data(), encodedSize());
-    text = text + m_decoder->flush();
-
+    text.append(m_decoder->flush());
     return text;
 }
 
 } // namespace blink