Add move constructor and assignment operator to RefPtr class

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 13078127885b7c8233cbb1e20515fdbd60125df8..094282f676ad541cf45d571d94306af75cb77eb3 100644
--- a/Source/wtf/text/WTFString.h
+++ b/Source/wtf/text/WTFString.h
@@ -123,13 +123,13 @@ public:
     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.