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

Source/core/editing/markup.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009, 2010, 2011 Google Inc. All rights reserved.
  * Copyright (C) 2011 Igalia S.L.
  * Copyright (C) 2011 Motorola Mobility. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
@@ skipping 739 matching lines @@
 
     if (string.isEmpty()) {
         paragraph->appendChild(createBlockPlaceholderElement(document));
         return;
     }
 
     ASSERT(string.find('\n') == kNotFound);
 
     Vector<String> tabList;
     string.split('\t', true, tabList);
-    String tabText = emptyString();
+    StringBuilder tabText;
     bool first = true;
     size_t numEntries = tabList.size();
     for (size_t i = 0; i < numEntries; ++i) {
         const String& s = tabList[i];
 
         // append the non-tab textual part
         if (!s.isEmpty()) {
             if (!tabText.isEmpty()) {
-                paragraph->appendChild(createTabSpanElement(document, tabText));
-                tabText = emptyString();
+                paragraph->appendChild(createTabSpanElement(document, tabText.toString()));
+                tabText.clear();
             }
             RefPtr<Node> textNode = document.createTextNode(stringWithRebalancedWhitespace(s, first, i + 1 == numEntries));
             paragraph->appendChild(textNode.release());
         }
 
         // there is a tab after every entry, except the last entry
         // (if the last character is a tab, the list gets an extra empty entry)
         if (i + 1 != numEntries)
             tabText.append('\t');
         else if (!tabText.isEmpty())
-            paragraph->appendChild(createTabSpanElement(document, tabText));
+            paragraph->appendChild(createTabSpanElement(document, tabText.toString()));
 
         first = false;
     }
 }
 
 bool isPlainTextMarkup(Node *node)
 {
     if (!node->isElementNode() || !node->hasTagName(divTag) || toElement(node)->hasAttributes())
         return false;
 
@@ skipping 288 matching lines @@
         return;
 
     RefPtr<Text> textNode = toText(node.get());
     RefPtr<Text> textNext = toText(next);
     textNode->appendData(textNext->data());
     if (textNext->parentNode()) // Might have been removed by mutation event.
         textNext->remove(exceptionState);
 }
 
 }