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

extensions/renderer/resources/guest_view/guest_view.js

Index: extensions/renderer/resources/guest_view/guest_view.js
diff --git a/extensions/renderer/resources/guest_view/guest_view.js b/extensions/renderer/resources/guest_view/guest_view.js
index 1f887a746b3572e23d2a7818d236c1f91422070f..939a0b18afcbb554969059aff812ff9e611beebb 100644
--- a/extensions/renderer/resources/guest_view/guest_view.js
+++ b/extensions/renderer/resources/guest_view/guest_view.js
@@ -45,6 +45,10 @@ function GuestViewImpl(guestView, viewType, guestInstanceId) {
   this.setupOnResize();
 }
 
+// Prevent GuestViewImpl inadvertently inheriting code from the global Object,
+// allowing a pathway for executing unintended user code execution.
+GuestViewImpl.prototype.__proto__ = null;
+
 // Possible states.
 GuestViewImpl.GuestState = {
   GUEST_STATE_START: 0,