Oilpan: consistently use IsGarbageCollectedMixin<> trait.

Source/modules/speech/SpeechRecognition.h

 /*
  * Copyright (C) 2012 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 copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 109 matching lines @@
     bool m_interimResults;
     unsigned long m_maxAlternatives;
 
     RawPtrWillBeMember<SpeechRecognitionController> m_controller;
     bool m_stoppedByActiveDOMObject;
     bool m_started;
     bool m_stopping;
     HeapVector<Member<SpeechRecognitionResult>> m_finalResults;
 };
 
-// FIXME: Oilpan: two GarbageCollectedLifetime-based subclasses introduces
-// ambiguity for WebPrivatePtr<T>'s LifeTimeOf<T> inference of lifetime.
-template<>
-class LifetimeOf<SpeechRecognition> {
-public:
-#if ENABLE(OILPAN)
-    static const LifetimeManagementType value = RefCountedGarbageCollectedLifetime;
-#else
-    static const LifetimeManagementType value = RefCountedLifetime;
-#endif
-};
-
 } // namespace blink
 
 #endif // SpeechRecognition_h