One accessibility tree per frame.

ui/accessibility/ax_tree_data.h

Index: ui/accessibility/ax_tree_data.h
diff --git a/ui/accessibility/ax_tree_data.h b/ui/accessibility/ax_tree_data.h
index 2079dfc98da6b1c6ecf034e058cfe63491130ee5..f780415be25cedc6ecc3792ded1cc6621a5d7676 100644
--- a/ui/accessibility/ax_tree_data.h
+++ b/ui/accessibility/ax_tree_data.h
@@ -38,6 +38,10 @@ struct AX_EXPORT AXTreeData {
   // 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;