One accessibility tree per frame.

third_party/WebKit/Source/web/WebAXObject.cpp

 /*
  * Copyright (C) 2009 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 106 matching lines @@
     if (isDetached())
         return -1;
 
     return m_private->axObjectID();
 }
 
 bool WebAXObject::updateLayoutAndCheckValidity()
 {
     if (!isDetached()) {
         Document* document = m_private->getDocument();
-        if (!document || !document->topDocument().view())
+        if (!document || !document->view())
             return false;
         document->view()->updateAllLifecyclePhases();
     }
 
     // Doing a layout can cause this object to be invalid, so check again.
     return !isDetached();
 }
 
 WebString WebAXObject::actionVerb() const
 {
@@ skipping 1370 matching lines @@
     m_private = object;
     return *this;
 }
 
 WebAXObject::operator AXObject*() const
 {
     return m_private.get();
 }
 
 } // namespace blink