Oilpan: remove unused support for removing FrameDestructionObservers.

Source/core/frame/FrameDestructionObserver.cpp

 /*
  * 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 21 matching lines @@
 
 FrameDestructionObserver::FrameDestructionObserver(LocalFrame* frame)
     : m_frame(nullptr)
 {
     observeFrame(frame);
 }
 
 #if !ENABLE(OILPAN)
 FrameDestructionObserver::~FrameDestructionObserver()
 {
-    observeFrame(0);
+    observeFrame(nullptr);
 }
 #endif
 
 void FrameDestructionObserver::observeFrame(LocalFrame* frame)
 {
+    ASSERT(!m_frame || !frame);
+#if !ENABLE(OILPAN)
     if (m_frame)
         m_frame->removeDestructionObserver(this);
+#endif
 
     m_frame = frame;
 
     if (m_frame)
         m_frame->addDestructionObserver(this);
 }
 
 #if !ENABLE(OILPAN)
 void FrameDestructionObserver::frameDestroyed()
 {
     m_frame = nullptr;
 }
 #endif
 
 void FrameDestructionObserver::willDetachFrameHost()
 {
     // Subclasses should override this function to handle this notification.
 }
 
 void FrameDestructionObserver::trace(Visitor* visitor)
 {
     visitor->trace(m_frame);
 }
 
 }