One accessibility tree per frame.

third_party/WebKit/Source/core/frame/FrameView.h

 /*
    Copyright (C) 1997 Martin Jones (mjones@kde.org)
              (C) 1998 Waldo Bastian (bastian@kde.org)
              (C) 1998, 1999 Torben Weis (weis@kde.org)
              (C) 1999 Lars Knoll (knoll@kde.org)
              (C) 1999 Antti Koivisto (koivisto@kde.org)
    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
 
    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Library General Public
@@ skipping 705 matching lines @@
     bool shouldUseCustomScrollbars(Element*& customScrollbarElement, LocalFrame*& customScrollbarFrame) const;
 
     // Returns true if a scrollbar needs to go from native -> custom or vice versa.
     bool needsScrollbarReconstruction() const;
 
     bool shouldIgnoreOverflowHidden() const;
 
     void updateScrollCorner();
 
     AXObjectCache* axObjectCache() const;
-    void removeFromAXObjectCache();
 
     void setLayoutSizeInternal(const IntSize&);
 
     bool adjustScrollbarExistence(ComputeScrollbarExistenceOption = FirstPass);
     void adjustScrollbarOpacity();
     void setScrollOffsetFromUpdateScrollbars(const DoubleSize&);
     bool visualViewportSuppliesScrollbars() const;
 
     IntRect rectToCopyOnScroll() const;
 
@@ skipping 191 matching lines @@
     if (m_visuallyNonEmptyPixelCount > visualPixelThreshold)
         setIsVisuallyNonEmpty();
 }
 
 DEFINE_TYPE_CASTS(FrameView, Widget, widget, widget->isFrameView(), widget.isFrameView());
 DEFINE_TYPE_CASTS(FrameView, ScrollableArea, scrollableArea, scrollableArea->isFrameView(), scrollableArea.isFrameView());
 
 } // namespace blink
 
 #endif // FrameView_h