Update from https://crrev.com/308996

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

Index: tools/clang/blink_gc_plugin/tests/heap/stubs.h
diff --git a/tools/clang/blink_gc_plugin/tests/heap/stubs.h b/tools/clang/blink_gc_plugin/tests/heap/stubs.h
index 7d646cdf85d2b4ee01beda24ded85b5ddef541eb..5281b126b49d36ca1ff8e14b6566333e470093cc 100644
--- a/tools/clang/blink_gc_plugin/tests/heap/stubs.h
+++ b/tools/clang/blink_gc_plugin/tests/heap/stubs.h
@@ -204,11 +204,15 @@ 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);
 };