Add WebScopedAXContext to allow a one-time snapshot of accessibility.

Source/core/dom/AXObjectCache.h

 /*
  * Copyright (C) 2003, 2006, 2007, 2008, 2009, 2010, 2011 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 123 matching lines @@
     typedef AXObjectCache* (*AXObjectCacheCreateFunction)(Document&);
     static void init(AXObjectCacheCreateFunction);
 
 protected:
     AXObjectCache();
 
 private:
     static AXObjectCacheCreateFunction m_createFunction;
 };
 
-class CORE_EXPORT ScopedAXObjectCache {
+class CORE_EXPORT ScopedAXObjectCache : public RefCounted<ScopedAXObjectCache> {
     WTF_MAKE_NONCOPYABLE(ScopedAXObjectCache);
 public:
     explicit ScopedAXObjectCache(Document&);
     ~ScopedAXObjectCache();
 
     AXObjectCache* get();
     AXObjectCache* operator->();
 
 private:
     Document& m_document;
     AXObjectCache* m_cache;
     bool m_isScoped;
 };
 
 }
 
 #endif