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

Source/core/dom/ActiveDOMObject.cpp

 /*
  * Copyright (C) 2008 Apple 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 30 matching lines @@
 }
 
 ActiveDOMObject::~ActiveDOMObject()
 {
     // ActiveDOMObject may be inherited by a sub-class whose life-cycle
     // exceeds that of the associated ExecutionContext. In those cases,
     // m_executionContext would/should have been nullified by
     // ContextLifecycleObserver::contextDestroyed() (which we implement /
     // inherit). Hence, we should ensure that this is not 0 before use it
     // here.
     if (!executionContext())

[haraken, 2015/03/16 23:44:22]

Is it safe to touch executionContext() here?

[sof, 2015/03/17 06:24:06]

We've deemed previously that null checking a (Weak)Member but dead reference is
safe; right? executionContext() fetches out m_lifecycleContext only.

[haraken, 2015/03/17 08:29:36]

Haven't we ended up with the point that it is unsafe if the ActiveDOMObject and
the ExecutionContext die together?

1) The ActiveDOMObject and the ExecutionContext die together.
2) ActiveDOMObject::m_executionContext is not cleared.
3) ~ExecutionContext() is called.
4) ~ActiveDOMObject() is called.

[sof, 2015/03/17 08:34:42]

It wouldn't be null at step 4, so the check would fail. How do you see this as
unsafe?

[haraken, 2015/03/17 08:36:22]

Yes, it wouldn't be null. So you'll call executionContext()->isContextThread()
for an ExecutionContext that has been already destructed?

         return;
 
     ASSERT(m_suspendIfNeededCalled);
+#if !ENABLE(OILPAN)

[sof, 2015/03/16 15:44:34]

Is this assert worth carrying over to Oilpan? I'm thinking not.

[haraken, 2015/03/16 23:44:22]

Agreed.

     ASSERT(executionContext()->isContextThread());
+#endif
 }
 
 void ActiveDOMObject::suspendIfNeeded()
 {
 #if ENABLE(ASSERT)
     ASSERT(!m_suspendIfNeededCalled);
     m_suspendIfNeededCalled = true;
 #endif
     if (ExecutionContext* context = executionContext())
         context->suspendActiveDOMObjectIfNeeded(this);
@@ skipping 27 matching lines @@
 
     if (context->activeDOMObjectsAreSuspended()) {
         suspend();
         return;
     }
 
     resume();
 }
 
 } // namespace blink