Implement RefCountedCopyable<T>.

third_party/WebKit/Source/platform/wtf/RefCounted.h

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010 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,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 146 matching lines @@
  protected:
 #if ENABLE(INSTANCE_COUNTER)
   RefCounted() { incrementInstanceCount<T>(static_cast<T*>(this)); }
 
   ~RefCounted() { decrementInstanceCount<T>(static_cast<T*>(this)); }
 #else
   RefCounted() {}
 #endif
 };
 
+// Unlike subclasses of RefCounted where the copy constructor must be
+// handwritten, subclasses of RefCountedCopyable can use the default
+// compiler-generated copy constructor.

[alancutter (OOO until 2018), 2017/04/03 12:07:45]

This wording is a bit too fancy. For speed of reading the point should be the
first thing conveyed. "Allows subclasses to use their default copy constructor,
...".

+template <typename T>
+class RefCountedCopyable : public RefCounted<T> {
+ protected:
+  RefCountedCopyable() = default;
+  RefCountedCopyable(const RefCountedCopyable&) : RefCounted<T>() {}
+};
+
 }  // namespace WTF
 
 using WTF::RefCounted;
+using WTF::RefCountedCopyable;
 
 #endif  // RefCounted_h