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

Source/core/fetch/CSSStyleSheetResource.cpp

Index: Source/core/fetch/CSSStyleSheetResource.cpp
diff --git a/Source/core/fetch/CSSStyleSheetResource.cpp b/Source/core/fetch/CSSStyleSheetResource.cpp
index 96ebcb71e5d6817c48d83cd733511605860c9160..be920489f280bb79685f86cff7c37bad9b17caeb 100755
--- a/Source/core/fetch/CSSStyleSheetResource.cpp
+++ b/Source/core/fetch/CSSStyleSheetResource.cpp
@@ -98,7 +98,7 @@ 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);