Add WTF::String::emptyString16Bit

Source/wtf/text/WTFString.h

 /*
  * (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012, 2013 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 637 matching lines @@
 }
 
 // StringHash is the default hash for String
 template<typename T> struct DefaultHash;
 template<> struct DefaultHash<String> {
     typedef StringHash Hash;
 };
 
 // Shared global empty string.
 WTF_EXPORT const String& emptyString();
+WTF_EXPORT const String& emptyString16Bit();
 WTF_EXPORT extern const String& xmlnsWithColon;
 
 } // namespace WTF
 
 WTF_ALLOW_MOVE_AND_INIT_WITH_MEM_FUNCTIONS(String);
 
 using WTF::CString;
 using WTF::KeepTrailingZeros;
 using WTF::StrictUTF8Conversion;
 using WTF::StrictUTF8ConversionReplacingUnpairedSurrogatesWithFFFD;
 using WTF::String;
 using WTF::emptyString;
+using WTF::emptyString16Bit;
 using WTF::append;
 using WTF::appendNumber;
 using WTF::charactersAreAllASCII;
 using WTF::charactersToIntStrict;
 using WTF::charactersToUIntStrict;
 using WTF::charactersToInt64Strict;
 using WTF::charactersToUInt64Strict;
 using WTF::charactersToIntPtrStrict;
 using WTF::charactersToInt;
 using WTF::charactersToUInt;
 using WTF::charactersToInt64;
 using WTF::charactersToUInt64;
 using WTF::charactersToIntPtr;
 using WTF::charactersToDouble;
 using WTF::charactersToFloat;
 using WTF::equal;
 using WTF::equalIgnoringCase;
 using WTF::find;
 using WTF::isAllSpecialCharacters;
 using WTF::isSpaceOrNewline;
 using WTF::reverseFind;
 
 #include "wtf/text/AtomicString.h"
 #endif // WTFString_h