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

Source/core/fetch/XSLStyleSheetResource.cpp

Index: Source/core/fetch/XSLStyleSheetResource.cpp
diff --git a/Source/core/fetch/XSLStyleSheetResource.cpp b/Source/core/fetch/XSLStyleSheetResource.cpp
old mode 100644
new mode 100755
index 053a445d456640d44516c275622671d2a230703c..36f853971f8cdd366334f5168dea5a64b2fb8349
--- a/Source/core/fetch/XSLStyleSheetResource.cpp
+++ b/Source/core/fetch/XSLStyleSheetResource.cpp
@@ -33,6 +33,7 @@
 #include "core/fetch/TextResourceDecoder.h"
 #include "platform/SharedBuffer.h"
 #include "wtf/Vector.h"
+#include "wtf/text/StringBuilder.h"
 
 namespace WebCore {
 
@@ -68,9 +69,11 @@ String XSLStyleSheetResource::encoding() const
 
 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);