One accessibility tree per frame.

third_party/WebKit/Source/core/html/HTMLDialogElement.cpp

 /*
  * 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 66 matching lines @@
 
     dialog->document().clearFocusedElement();
 }
 
 static void inertSubtreesChanged(Document& document)
 {
     // When a modal dialog opens or closes, nodes all over the accessibility
     // tree can change inertness which means they must be added or removed from
     // the tree. The most foolproof way is to clear the entire tree and rebuild
     // it, though a more clever way is probably possible.
-    Document& topDocument = document.topDocument();
-    topDocument.clearAXObjectCache();
-    if (AXObjectCache* cache = topDocument.axObjectCache())
-        cache->childrenChanged(&topDocument);
+    document.clearAXObjectCache();
+    if (AXObjectCache* cache = document.axObjectCache())
+        cache->childrenChanged(&document);
 }
 
 inline HTMLDialogElement::HTMLDialogElement(Document& document)
     : HTMLElement(dialogTag, document)
     , m_centeringMode(NotCentered)
     , m_centeredPosition(0)
     , m_returnValue("")
 {
     UseCounter::count(document, UseCounter::DialogElement);
 }
@@ skipping 102 matching lines @@
 {
     if (event->type() == EventTypeNames::cancel) {
         closeDialog();
         event->setDefaultHandled();
         return;
     }
     HTMLElement::defaultEventHandler(event);
 }
 
 } // namespace blink