[Extensions] More bindings hardening

chrome/renderer/resources/extensions/automation_custom_bindings.js

Index: chrome/renderer/resources/extensions/automation_custom_bindings.js
diff --git a/chrome/renderer/resources/extensions/automation_custom_bindings.js b/chrome/renderer/resources/extensions/automation_custom_bindings.js
index 82a1e6c8bf5fe0bde75916f1414605dc178eed49..7321710fb7d5a08761de9e5dca8a84438758b75d 100644
--- a/chrome/renderer/resources/extensions/automation_custom_bindings.js
+++ b/chrome/renderer/resources/extensions/automation_custom_bindings.js
@@ -324,11 +324,13 @@ automationInternal.onAccessibilityTreeDestroyed.addListener(function(id) {
   DestroyAccessibilityTree(id);
 });
 
-exports.binding = automation.generate();
-
+var binding = automation.generate();
 // Add additional accessibility bindings not specified in the automation IDL.
 // Accessibility and automation share some APIs (see
 // ui/accessibility/ax_enums.idl).
 forEach(schema, function(k, v) {
-  exports.binding[k] = v;
+  binding[k] = v;
 });
+
+exports.$set('binding', binding);
+