One accessibility tree per frame.

content/browser/frame_host/frame_tree.cc

Index: content/browser/frame_host/frame_tree.cc
diff --git a/content/browser/frame_host/frame_tree.cc b/content/browser/frame_host/frame_tree.cc
index ceb598592d904feac325fe92b0e8e8347aa2fbce..90e3a7265c365815561086c12da3adbb8aafccd5 100644
--- a/content/browser/frame_host/frame_tree.cc
+++ b/content/browser/frame_host/frame_tree.cc
@@ -327,6 +327,11 @@ void FrameTree::SetFocusedFrame(FrameTreeNode* node, SiteInstance* source) {
 
   focused_frame_tree_node_id_ = node->frame_tree_node_id();
   node->DidFocus();
+
+  // The accessibility tree data for the root of the frame tree keeps
+  // track of the focused frame too, so update that every time the
+  // focused frame changes.
+  root()->current_frame_host()->UpdateAXTreeData();
 }
 
 void FrameTree::SetFrameRemoveListener(