Oilpan: catch some self-referential leaks (main thread.)

Source/platform/heap/PersistentNode.h

Index: Source/platform/heap/PersistentNode.h
diff --git a/Source/platform/heap/PersistentNode.h b/Source/platform/heap/PersistentNode.h
index 338adf2b4ca7ba114ac580986d81928571d3d502..da8f3655f63fc72ca1d384fe2487d6b8f5a3ca56 100644
--- a/Source/platform/heap/PersistentNode.h
+++ b/Source/platform/heap/PersistentNode.h
@@ -82,6 +82,11 @@ public:
         return m_self;
     }
 
+    TraceCallback traceCallback() const
+    {
+        return m_trace;
+    }
+
 private:
     // If this PersistentNode is in use:
     //   - m_self points to the corresponding Persistent handle.
@@ -144,6 +149,10 @@ public:
     void tracePersistentNodes(Visitor*);
     int numberOfPersistents();
 
+#ifndef NDEBUG
+    void dumpLivePersistents();
+#endif
+
 private:
     friend CrossThreadPersistentRegion;