Oilpan: Move MainThreadTaskRunner into Oilpan heap.

Source/core/dom/Document.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 1311 matching lines @@
     Length m_viewportDefaultMinWidth;
 
     ReferrerPolicy m_referrerPolicy;
 
     DocumentTiming m_documentTiming;
     RefPtrWillBeMember<MediaQueryMatcher> m_mediaQueryMatcher;
     bool m_writeRecursionIsTooDeep;
     unsigned m_writeRecursionDepth;
 
     RefPtrWillBeMember<ScriptedAnimationController> m_scriptedAnimationController;
-    OwnPtr<MainThreadTaskRunner> m_taskRunner;
+    OwnPtrWillBeMember<MainThreadTaskRunner> m_taskRunner;
     OwnPtrWillBeMember<TextAutosizer> m_textAutosizer;
 
     RefPtrWillBeMember<CustomElementRegistrationContext> m_registrationContext;
     RefPtrWillBeMember<CustomElementMicrotaskRunQueue> m_customElementMicrotaskRunQueue;
 
     void elementDataCacheClearTimerFired(Timer<Document>*);
     Timer<Document> m_elementDataCacheClearTimer;
 
     OwnPtrWillBeMember<ElementDataCache> m_elementDataCache;
 
@@ skipping 76 matching lines @@
 DEFINE_TYPE_CASTS(TreeScope, Document, document, true, true);
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 CORE_EXPORT void showLiveDocumentInstances();
 #endif
 
 #endif // Document_h