One accessibility tree per frame.

ui/accessibility/ax_tree_data.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_ACCESSIBILITY_AX_TREE_DATA_H_
 #define UI_ACCESSIBILITY_AX_TREE_DATA_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 20 matching lines @@
 
   // This is a simple serializable struct. All member variables should be
   // public and copyable.
 
   // The globally unique ID of this accessibility tree.
   int32_t tree_id;
 
   // The ID of the accessibility tree that this tree is contained in, if any.
   int32_t parent_tree_id;
 
+  // The ID of the accessibility tree that has focus. This is typically set
+  // on the root frame in a frame tree.
+  int32_t focused_tree_id;
+
   // Attributes specific to trees that are web frames.
   std::string url;
   std::string title;
   std::string mimetype;
   std::string doctype;
   bool loaded;
   float loading_progress;
 
   // The node with keyboard focus within this tree, if any, or -1 if no node
   // in this tree has focus.
   int32_t focus_id;
 
   // The current text selection within this tree, if any, expressed as the
   // node ID and character offset of the anchor (selection start) and focus
   // (selection end).
   int32_t sel_anchor_object_id;
   int32_t sel_anchor_offset;
   int32_t sel_focus_object_id;
   int32_t sel_focus_offset;
 };
 
 AX_EXPORT bool operator==(const AXTreeData& lhs, const AXTreeData& rhs);
 AX_EXPORT bool operator!=(const AXTreeData& lhs, const AXTreeData& rhs);
 
 }  // namespace ui
 
 #endif  // UI_ACCESSIBILITY_AX_TREE_DATA_H_