Oilpan: Add a comment to a Persistent data member in DatabaseSync.

Source/modules/webdatabase/DatabaseSync.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 70 matching lines @@
 #if ENABLE(OILPAN)
     class TransactionObserver {
     public:
         explicit TransactionObserver(SQLTransactionSync& transaction) : m_transaction(transaction) { }
         ~TransactionObserver();
 
     private:
         SQLTransactionSync& m_transaction;
     };
 
+    // Need a Persistent field because a HeapHashMap entry should be removed
+    // just after a SQLTransactionSync becomes untraceable. If this field was a
+    // Member<>, we could not assume the destruction order of DatabaseSync,
+    // SQLTransactionSync, and the field. We can not make the field static
+    // because multiple worker threads create SQLTransactionSync.
     GC_PLUGIN_IGNORE("http://crbug.com/353083")
     PersistentHeapHashMap<WeakMember<SQLTransactionSync>, OwnPtr<TransactionObserver> > m_observers;
 #endif
 
     String m_lastErrorMessage;
 
     friend class DatabaseManager;
     friend class DatabaseServer; // FIXME: remove this when the backend has been split out.
 };
 
 } // namespace WebCore
 
 #endif