Remove ResourceFetcher's access to a real FetchContext at frame detach time

Source/core/fetch/ResourceFetcher.h

 /*
     Copyright (C) 1998 Lars Knoll (knoll@mpi-hd.mpg.de)
     Copyright (C) 2001 Dirk Mueller <mueller@kde.org>
     Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
     Copyright (C) 2009 Torch Mobile Inc. http://www.torchmobile.com/
 
     This library is free software; you can redistribute it and/or
     modify it under the terms of the GNU Library General Public
     License as published by the Free Software Foundation; either
     version 2 of the License, or (at your option) any later version.
@@ skipping 85 matching lines @@
     typedef HashMap<String, ResourcePtr<Resource>> DocumentResourceMap;
     const DocumentResourceMap& allResources() const { return m_documentResources; }
 
     bool autoLoadImages() const { return m_autoLoadImages; }
     void setAutoLoadImages(bool);
 
     void setImagesEnabled(bool);
 
     bool shouldDeferImageLoad(const KURL&) const;
 
-    FetchContext& context() const { return *m_context.get(); }
+    FetchContext& context() const { return m_context ? *m_context.get() : FetchContext::nullInstance(); }
+    void clearContext() { m_context.clear(); }
 
     void garbageCollectDocumentResources();
 
     int requestCount() const;
 
     bool isPreloaded(const KURL&) const;
     void clearPreloads();
     void preload(Resource::Type, FetchRequest&, const String& charset);
     void printPreloadStats();
 
@@ skipping 135 matching lines @@
             m_loader->m_allowStaleResources = m_previousState;
     }
 private:
     RawPtrWillBeMember<ResourceFetcher> m_loader;
     bool m_previousState;
 };
 
 } // namespace blink
 
 #endif // ResourceFetcher_h