Update from https://crrev.com/308996

tools/clang/blink_gc_plugin/tests/heap/stubs.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef HEAP_STUBS_H_
 #define HEAP_STUBS_H_
 
 #include "stddef.h"
 
 #define WTF_MAKE_FAST_ALLOCATED                 \
@@ skipping 186 matching lines @@
 template<typename T>
 class HeapHashCountedSet : public HashCountedSet<T, void, void, HeapAllocator> {
 };
 
 template<typename K, typename V>
 class HeapHashMap : public HashMap<K, V, void, void, void, HeapAllocator> { };
 
 template<typename T>
 class PersistentHeapVector : public Vector<T, 0, HeapAllocator> { };
 
-class Visitor {
+template <typename Derived>
+class VisitorHelper {
 public:
     template<typename T>
     void trace(const T&);
+};
 
+class Visitor : public VisitorHelper<Visitor> {
+ public:
     template<typename T, void (T::*method)(Visitor*)>
     void registerWeakMembers(const T* obj);
 };
 
 class GarbageCollectedMixin {
     virtual void adjustAndMark(Visitor*) const = 0;
     virtual bool isAlive(Visitor*) const = 0;
     virtual void trace(Visitor*) { }
 };
 
@@ skipping 16 matching lines @@
 namespace WTF {
 
 template<typename T>
 struct VectorTraits<blink::Member<T> > {
     static const bool needsDestruction = false;
 };
 
 }
 
 #endif