Add move constructor and assignment operator to RefPtr class

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 105 matching lines @@
     String(const char* characters, unsigned length);
 
     // Construct a string with latin1 data, from a null-terminated source.
     String(const LChar* characters);
     String(const char* characters);
 
     // Construct a string referencing an existing StringImpl.
     String(StringImpl* impl) : m_impl(impl) { }
     String(PassRefPtr<StringImpl> impl) : m_impl(impl) { }
 
-#if COMPILER_SUPPORTS(CXX_RVALUE_REFERENCES)
-    // We have to declare the copy constructor and copy assignment operator as well, otherwise
-    // they'll be implicitly deleted by adding the move constructor and move assignment operator.
-    String(const String& other) : m_impl(other.m_impl) { }
-    String(String&& other) : m_impl(other.m_impl.release()) { }
-    String& operator=(const String& other) { m_impl = other.m_impl; return *this; }
-    String& operator=(String&& other) { m_impl = other.m_impl.release(); return *this; }
+#if COMPILER_SUPPORTS(CXX_RVALUE_REFERENCES) && COMPILER_SUPPORTS(CXX_DEFAULTED_FUNCTIONS)
+    // We have to explicitly declare the move constructor and move copy assignment operator. The compiler does
+    // not generate them because the class has an explicitly defined destructor.
+    String(const String&) = default;
+    String(String&&) = default;
+    String& operator=(const String&) = default;
+    String& operator=(String&&) = default;
 #endif
 
     // Inline the destructor.
     ALWAYS_INLINE ~String() { }
 
     void swap(String& o) { m_impl.swap(o.m_impl); }
 
     template<typename CharType>
     static String adopt(StringBuffer<CharType>& buffer)
     {
@@ skipping 558 matching lines @@
 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