Revert of [oilpan]: Remove support for tracing off-heap hashmaps.

Source/core/dom/IdTargetObserverRegistry.cpp

 /*
  * 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:
  * 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 19 matching lines @@
 
 namespace blink {
 
 PassOwnPtrWillBeRawPtr<IdTargetObserverRegistry> IdTargetObserverRegistry::create()
 {
     return adoptPtrWillBeNoop(new IdTargetObserverRegistry());
 }
 
 void IdTargetObserverRegistry::trace(Visitor* visitor)
 {
-#if ENABLE(OILPAN)
     visitor->trace(m_registry);
     visitor->trace(m_notifyingObserversInSet);
-#endif
 }
 
 void IdTargetObserverRegistry::addObserver(const AtomicString& id, IdTargetObserver* observer)
 {
     if (id.isEmpty())
         return;
 
     IdToObserverSetMap::AddResult result = m_registry.add(id.impl(), nullptr);
     if (result.isNewEntry)
         result.storedValue->value = adoptPtrWillBeNoop(new ObserverSet());
@@ skipping 38 matching lines @@
 
 bool IdTargetObserverRegistry::hasObservers(const AtomicString& id) const
 {
     if (id.isEmpty() || m_registry.isEmpty())
         return false;
     ObserverSet* set = m_registry.get(id.impl());
     return set && !set->isEmpty();
 }
 
 } // namespace blink