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

Source/core/dom/Document.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 188 matching lines @@
     virtual ~DocumentVisibilityObserver();
 
     virtual void didChangeVisibilityState(PageVisibilityState) = 0;
 
     // Classes that inherit Node and DocumentVisibilityObserver must have a
     // virtual override of Node::didMoveToNewDocument that calls
     // DocumentVisibilityObserver::setDocument
     void setObservedDocument(Document&);
 
 protected:
-    void trace(Visitor*);
+    virtual void trace(Visitor*);
 
 private:
     void registerObserver(Document&);
     void unregisterObserver();
 
     RawPtrWillBeMember<Document> m_document;
 };
 
 class Document : public ContainerNode, public TreeScope, public SecurityContext, public ExecutionContext, public ExecutionContextClient
     , public DocumentSupplementable, public LifecycleContext<Document> {
@@ skipping 1201 matching lines @@
 Node* eventTargetNodeForDocument(Document*);
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLiveDocumentInstances();
 #endif
 
 #endif // Document_h