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

Source/core/fileapi/Blob.h

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 75 matching lines @@
     // Only true for File instances that are backed by platform files.
     virtual bool hasBackingFile() const { return false; }
     bool hasBeenClosed() const { return m_hasBeenClosed; }
 
     // Used by the JavaScript Blob and File constructors.
     virtual void appendTo(BlobData&) const;
 
     // URLRegistrable to support PublicURLs.
     virtual URLRegistry& registry() const OVERRIDE FINAL;
 
-    void trace(Visitor*) { }
+    virtual void trace(Visitor*) { }
 
 protected:
     explicit Blob(PassRefPtr<BlobDataHandle>);
 
     static void clampSliceOffsets(long long size, long long& start, long long& end);
 private:
     Blob();
 
     RefPtr<BlobDataHandle> m_blobDataHandle;
     bool m_hasBeenClosed;
 };
 
 } // namespace blink
 
 #endif // Blob_h