Hide AXPlatformNode on ChromeOS.

ui/base/ui_features.gni

Index: ui/base/ui_features.gni
diff --git a/ui/base/ui_features.gni b/ui/base/ui_features.gni
index b563b2b2b13ae678cc8e08625ba7f7a549f7cf08..2dca6a502ca16609bda13daae1593f406271ff60 100644
--- a/ui/base/ui_features.gni
+++ b/ui/base/ui_features.gni
@@ -2,12 +2,17 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/config/ui.gni")
+
 declare_args() {
   # Optional system library.
   use_xkbcommon = false
 
   # Whether the entire browser uses toolkit-views on Mac instead of Cocoa.
   mac_views_browser = false
+
+  # Whether the platform provides a native accessibility toolkit.
+  has_native_accessibility = use_atk || is_win || is_mac
 }
 
 enable_hidpi = is_mac || is_win || is_linux