StringBuilder::append should not use reserveCapacity.

third_party/WebKit/Source/wtf/text/StringBuilder.cpp

 /*
  * Copyright (C) 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2012 Google Inc. 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
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 182 matching lines @@
     if (m_is8Bit) {
         ensureBuffer8(length);
         m_string = String();
         m_buffer8->append(characters, length);
         m_length += length;
         return;
     }
 
     ensureBuffer16(length);
     m_string = String();
-    m_buffer16->reserveCapacity(m_buffer16->size() + length);
-    for (size_t i = 0; i < length; ++i)
-        m_buffer16->uncheckedAppend(characters[i]);
+    m_buffer16->append(characters, length);
     m_length += length;
 }
 
 template<typename IntegerType>
 static void appendIntegerInternal(StringBuilder& builder, IntegerType input)
 {
     IntegerToStringConverter<IntegerType> converter(input);
     builder.append(converter.characters8(), converter.length());
 }
 
@@ skipping 27 matching lines @@
     appendIntegerInternal(*this, number);
 }
 
 void StringBuilder::appendNumber(double number, unsigned precision, TrailingZerosTruncatingPolicy trailingZerosTruncatingPolicy)
 {
     NumberToStringBuffer buffer;
     append(numberToFixedPrecisionString(number, precision, buffer, trailingZerosTruncatingPolicy == TruncateTrailingZeros));
 }
 
 } // namespace WTF