Set prototype for GuestView*.prototype to null.

extensions/renderer/resources/guest_view/guest_view_events.js

Index: extensions/renderer/resources/guest_view/guest_view_events.js
diff --git a/extensions/renderer/resources/guest_view/guest_view_events.js b/extensions/renderer/resources/guest_view/guest_view_events.js
index e3ccde1a6c65c80541c333093677457cbbfcb5ab..9f6bdfa01718310ee11d6d1055fb06f27e77ece6 100644
--- a/extensions/renderer/resources/guest_view/guest_view_events.js
+++ b/extensions/renderer/resources/guest_view/guest_view_events.js
@@ -27,6 +27,12 @@ function GuestViewEvents(view) {
   this.setupEvents();
 }
 
+// Prevent GuestViewEvents inadvertently inheritng code from the global Object,
+// allowing a pathway for unintended execution of user code.
+// TODO(wjmaclean): Use utils.expose() here instead, track down other issues
+// of Object inheritance. https://crbug.com/701034
+GuestViewEvents.prototype.__proto__ = null;
+
 // |GuestViewEvents.EVENTS| is a dictionary of extension events to be listened
 //     for, which specifies how each event should be handled. The events are
 //     organized by name, and by default will be dispatched as DOM events with