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

Source/modules/webdatabase/DatabaseSync.h

Index: Source/modules/webdatabase/DatabaseSync.h
diff --git a/Source/modules/webdatabase/DatabaseSync.h b/Source/modules/webdatabase/DatabaseSync.h
index f1317c36cb352ab0dbb02cb1f44a7fc390472f99..d3cea75ca7088fbf06f08c9572f4e85faaf2a9b8 100644
--- a/Source/modules/webdatabase/DatabaseSync.h
+++ b/Source/modules/webdatabase/DatabaseSync.h
@@ -88,6 +88,11 @@ 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