Don't allow GuestView JS objects to inherit global prototype.

extensions/renderer/resources/guest_view/guest_view_container.js

Index: extensions/renderer/resources/guest_view/guest_view_container.js
diff --git a/extensions/renderer/resources/guest_view/guest_view_container.js b/extensions/renderer/resources/guest_view/guest_view_container.js
index 8cfc8cb441afa19109e320c0a6405f79e9878a85..efec02adb3a64303096e849bd0ec5888fa71499a 100644
--- a/extensions/renderer/resources/guest_view/guest_view_container.js
+++ b/extensions/renderer/resources/guest_view/guest_view_container.js
@@ -31,6 +31,10 @@ function GuestViewContainer(element, viewType) {
   GuestViewInternalNatives.RegisterView(this.viewInstanceId, this, viewType);
 }
 
+// Prevent GuestViewContainer inadvertently inheriting code from the global
+// Object, allowing a pathway for executing unintended user code execution.
+GuestViewContainer.prototype.__proto__ = null;
+
 // Forward public API methods from |proto| to their internal implementations.
 GuestViewContainer.forwardApiMethods = function(proto, apiMethods) {
   var createProtoHandler = function(m) {