Revert of Remove user-defined move constructor and assignment operator from WTF::String

Source/wtf/text/WTFString.h

Index: Source/wtf/text/WTFString.h
diff --git a/Source/wtf/text/WTFString.h b/Source/wtf/text/WTFString.h
index c7650b50e9096329045deedd181dc733ef0e363d..13078127885b7c8233cbb1e20515fdbd60125df8 100644
--- a/Source/wtf/text/WTFString.h
+++ b/Source/wtf/text/WTFString.h
@@ -122,6 +122,15 @@
     // 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; }
+#endif
 
     // Inline the destructor.
     ALWAYS_INLINE ~String() { }