Oilpan: Ensure that classes with virtual trace methods always have vtables for their left-most base…

Source/core/dom/custom/CustomElementMicrotaskQueueBase.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 CustomElementMicrotaskQueueBase_h
 #define CustomElementMicrotaskQueueBase_h
 
 #include "core/dom/custom/CustomElementMicrotaskStep.h"
 #include "platform/heap/Handle.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/PassRefPtr.h"
 #include "wtf/RefCounted.h"
 #include "wtf/RefPtr.h"
 #include "wtf/Vector.h"
 
 namespace blink {
 
 class CustomElementMicrotaskQueueBase : public RefCountedWillBeGarbageCollectedFinalized<CustomElementMicrotaskQueueBase> {
     WTF_MAKE_NONCOPYABLE(CustomElementMicrotaskQueueBase);
 public:
     virtual ~CustomElementMicrotaskQueueBase() { }
 
     bool isEmpty() const { return m_queue.isEmpty(); }
     void dispatch();
 
-    void trace(Visitor*);
+    virtual void trace(Visitor*);
 
 #if !defined(NDEBUG)
     void show(unsigned indent);
 #endif
 
 protected:
     CustomElementMicrotaskQueueBase() : m_inDispatch(false) { }
     virtual void doDispatch() = 0;
 
     WillBeHeapVector<OwnPtrWillBeMember<CustomElementMicrotaskStep> > m_queue;
     bool m_inDispatch;
 };
 
 }
 
 #endif // CustomElementMicrotaskQueueBase_h