Set prototype for GuestView*.prototype to null.

extensions/renderer/resources/guest_view/web_view/web_view_action_requests.js

Index: extensions/renderer/resources/guest_view/web_view/web_view_action_requests.js
diff --git a/extensions/renderer/resources/guest_view/web_view/web_view_action_requests.js b/extensions/renderer/resources/guest_view/web_view/web_view_action_requests.js
index 5f4bf2c06078af6b2a4afc93cef0eb790cb73e40..6ca63a775c076f8acb3e2d8aa86c35625005b914 100644
--- a/extensions/renderer/resources/guest_view/web_view/web_view_action_requests.js
+++ b/extensions/renderer/resources/guest_view/web_view/web_view_action_requests.js
@@ -37,6 +37,12 @@ function WebViewActionRequest(webViewImpl, event, webViewEvent, interfaceName) {
   }
 }
 
+// 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
+WebViewActionRequest.prototype.__proto__ = null;
+
 // Performs the default action for the request.
 WebViewActionRequest.prototype.defaultAction = function() {
   // Do nothing if the action has already been taken or the requester is