Oilpan: have LifetimeNotifier<T> track its observers weakly.

Source/bindings/core/v8/ScriptPromiseResolver.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "config.h"
 #include "bindings/core/v8/ScriptPromiseResolver.h"
 
 #include "bindings/core/v8/V8RecursionScope.h"
 #include "platform/LifecycleObserver.h"
 
 namespace blink {
 
 ScriptPromiseResolver::ScriptPromiseResolver(ScriptState* scriptState)
     : ActiveDOMObject(scriptState->executionContext())
     , m_state(Pending)
     , m_scriptState(scriptState)
     , m_mode(Default)
     , m_timer(this, &ScriptPromiseResolver::onTimerFired)
     , m_resolver(scriptState)
 #if ENABLE(ASSERT)
     , m_isPromiseCalled(false)
 #endif
 {
     if (executionContext()->activeDOMObjectsAreStopped()) {
         m_state = ResolvedOrRejected;
         m_resolver.clear();
     }
 #if ENABLE(OILPAN) && ENABLE(ASSERT)
-    // A prefinalizer has already been registered (as a LifecycleObserver);
-    // remove it and register a combined one, as the infrastructure doesn't
-    // support multiple prefinalizers for an object.
-    //
-    // FIXME: Oilpan: remove LifecycleObserver's need for a prefinalizer,
-    // and as a consequence, this unregistration step. If the former is
-    // independently removed, the unregisterPreFinalizer() call will assert.
-    ThreadState::current()->unregisterPreFinalizer(*static_cast<LifecycleObserver*>(this));
     ThreadState::current()->registerPreFinalizer(*this);
 #endif
 }
 
 void ScriptPromiseResolver::suspend()
 {
     m_timer.stop();
 }
 
 void ScriptPromiseResolver::resume()
@@ skipping 61 matching lines @@
         deref();
     }
 }
 
 DEFINE_TRACE(ScriptPromiseResolver)
 {
     ActiveDOMObject::trace(visitor);
 }
 
 } // namespace blink