Oilpan: Prepare to make ExecutionContext GarbageCollectedMixin.

Source/modules/webdatabase/SQLCallbackWrapper.h

 /*
  * Copyright (C) 2011 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 38 matching lines @@
         , m_executionContext(m_callback ? executionContext : 0)
     {
         ASSERT(!m_callback || (m_executionContext.get() && m_executionContext->isContextThread()));
     }
 
     ~SQLCallbackWrapper()
     {
         clear();
     }
 
-    // FIXME: Oilpan: Trace m_executionContext.
-    void trace(Visitor* visitor) { }
+    void trace(Visitor* visitor) { visitor->trace(m_executionContext); }
 
     void clear()
     {
 #if ENABLE(OILPAN)
         // It's safe to call ~T in non-context-thread.
         // Implementation classes of ExecutionContext are on-heap. Their
         // destructors are called in their owner threads.
         MutexLocker locker(m_mutex);
         m_callback.clear();
         m_executionContext.clear();
@@ skipping 52 matching lines @@
             : m_callbackToRelease(callbackToRelease)
         {
         }
 
         OwnPtr<T> m_callbackToRelease;
     };
 #endif
 
     Mutex m_mutex;
     OwnPtr<T> m_callback;
-    RefPtr<ExecutionContext> m_executionContext;
+    RefPtrWillBeMember<ExecutionContext> m_executionContext;
 };
 
 } // namespace WebCore
 
 #endif // SQLCallbackWrapper_h