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

Source/core/dom/ExecutionContext.h

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  * 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
@@ skipping 34 matching lines @@
 class DOMTimerCoordinator;
 class ErrorEvent;
 class EventQueue;
 class EventTarget;
 class ExecutionContextTask;
 class LocalDOMWindow;
 class PublicURLManager;
 class SecurityOrigin;
 class ScriptCallStack;
 
-class ExecutionContext
-    : public ContextLifecycleNotifier, public WillBeHeapSupplementable<ExecutionContext> {
+class ExecutionContext : public ContextLifecycleNotifier, public WillBeHeapSupplementable<ExecutionContext> {
 public:
     DECLARE_VIRTUAL_TRACE();
 
     virtual bool isDocument() const { return false; }
     virtual bool isWorkerGlobalScope() const { return false; }
     virtual bool isDedicatedWorkerGlobalScope() const { return false; }
     virtual bool isSharedWorkerGlobalScope() const { return false; }
     virtual bool isServiceWorkerGlobalScope() const { return false; }
     virtual bool isJSExecutionForbidden() const { return false; }
 
@@ skipping 96 matching lines @@
     // Counter that keeps track of how many window interaction calls are allowed
     // for this ExecutionContext. Callers are expected to call
     // |allowWindowInteraction()| and |consumeWindowInteraction()| in order to
     // increment and decrement the counter.
     int m_windowInteractionTokens;
 };
 
 } // namespace blink
 
 #endif // ExecutionContext_h