Avoid extra copy of CSS data.

Source/core/fetch/TextResource.cpp

Index: Source/core/fetch/TextResource.cpp
diff --git a/Source/core/fetch/TextResource.cpp b/Source/core/fetch/TextResource.cpp
index 0f1d56d8529c64e1d3b9a2e0a803128710a4c41b..5ec44e320521e772a3ed5da960bfb08142eb9c8f 100644
--- a/Source/core/fetch/TextResource.cpp
+++ b/Source/core/fetch/TextResource.cpp
@@ -35,8 +35,7 @@ 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;
 }