On the Mac, AX_EVENT_LOAD_COMPLETE should only be fired on the top document.

content/browser/accessibility/browser_accessibility_manager.h

 // Copyright (c) 2012 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 CONTENT_BROWSER_ACCESSIBILITY_BROWSER_ACCESSIBILITY_MANAGER_H_
 #define CONTENT_BROWSER_ACCESSIBILITY_BROWSER_ACCESSIBILITY_MANAGER_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 361 matching lines @@
   }
 
   // If this BrowserAccessibilityManager is a child frame or guest frame,
   // return the BrowserAccessibilityManager from the highest ancestor frame
   // in the frame tree.
   BrowserAccessibilityManager* GetRootManager();
 
   // Returns the BrowserAccessibilityDelegate from |GetRootManager|, above.
   BrowserAccessibilityDelegate* GetDelegateFromRootManager();
 
+  // Returns whether this is the top document.
+  bool IsRootTree();
+
   // Get a snapshot of the current tree as an AXTreeUpdate.
   ui::AXTreeUpdate SnapshotAXTreeForTesting();
 
  protected:
   BrowserAccessibilityManager(
       BrowserAccessibilityDelegate* delegate,
       BrowserAccessibilityFactory* factory);
 
   BrowserAccessibilityManager(
       const ui::AXTreeUpdate& initial_tree,
@@ skipping 72 matching lines @@
   // node within that parent tree. It's computed as needed and cached for
   // speed so that it can be accessed quickly if it hasn't changed.
   int parent_node_id_from_parent_tree_;
 
   DISALLOW_COPY_AND_ASSIGN(BrowserAccessibilityManager);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_ACCESSIBILITY_BROWSER_ACCESSIBILITY_MANAGER_H_