Blink GC plugin: Ensure that classes with virtual trace methods always have vtables for their left-…

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 198 matching lines @@
     template<typename T>
     void trace(const T&);
 
     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*) { }
 };
 
 template<typename T>
 struct TraceIfNeeded {
     static void trace(Visitor*, T*);
 };
 
 // blink::ScriptWrappable receives special treatment
 // so as to allow it to be used together with GarbageCollected<T>,
 // even when its user-declared destructor is provided.
 // As it is with Oilpan disabled.
 class ScriptWrappable {
 public:
     ~ScriptWrappable() { /* user-declared, thus, non-trivial */ }
 };
 
 }
 
 namespace WTF {
 
 template<typename T>
 struct VectorTraits<blink::Member<T> > {
     static const bool needsDestruction = false;
 };
 
 }
 
 #endif