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

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 38 matching lines @@
 class LocalDOMWindow;
 class ErrorEvent;
 class EventQueue;
 class ExecutionContextTask;
 class ScriptState;
 class PublicURLManager;
 class SecurityOrigin;
 class ScriptCallStack;
 
 class ExecutionContext
-    : public LifecycleContext<ExecutionContext>
-    , public WillBeHeapSupplementable<ExecutionContext> {
+    : public WillBeGarbageCollectedMixin
+    , public LifecycleContext<ExecutionContext>
+    , public Supplementable<ExecutionContext> {
 public:
-    virtual void trace(Visitor*) OVERRIDE;
+    virtual void trace(Visitor*);
 
     // Delegating to ExecutionContextClient
     bool isDocument() const { return m_client && m_client->isDocument(); }
     bool isWorkerGlobalScope() const { return m_client && m_client->isWorkerGlobalScope(); }
     bool isJSExecutionForbidden() { return m_client && m_client->isJSExecutionForbidden(); }
     SecurityOrigin* securityOrigin() const;
     ContentSecurityPolicy* contentSecurityPolicy() const;
     const KURL& url() const;
     KURL completeURL(const String& url) const;
     void disableEval(const String& errorMessage);
@@ skipping 96 matching lines @@
 
     // The location of this member is important; to make sure contextDestroyed() notification on
     // ExecutionContext's members (notably m_timeouts) is called before they are destructed,
     // m_lifecycleNotifer should be placed *after* such members.
     OwnPtr<ContextLifecycleNotifier> m_lifecycleNotifier;
 };
 
 } // namespace blink
 
 #endif // ExecutionContext_h