Reject connection attempts without prompt if target app doesn't handle the event anyway.

chrome/common/extensions/api/extension.json

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
   {
     "namespace": "extension",
     "description": "The <code>chrome.extension</code> API has utilities that can be used by any extension page. It includes support for exchanging messages between an extension and its content scripts or between extensions, as described in detail in <a href='messaging'>Message Passing</a>.",
     "compiler_options": {
       "implemented_in": "chrome/browser/extensions/api/module/module.h"
@@ skipping 185 matching lines @@
         "parameters": [
           {"name": "request", "type": "any", "optional": true, "description": "The request sent by the calling script."},
           {"name": "sender", "$ref": "runtime.MessageSender" },
           {"name": "sendResponse", "type": "function", "description": "Function to call (at most once) when you have a response. The argument should be any JSON-ifiable object, or undefined if there is no response. If you have more than one <code>onRequest</code> listener in the same document, then only one may send a response." }
         ]
       },
       {
         "name": "onRequestExternal",
         "deprecated": "Please use $(ref:runtime.onMessageExternal).",
         "type": "function",
-        "options": {
-          "unmanaged": true
-        },
         "description": "Fired when a request is sent from another extension.",
         "parameters": [
           {"name": "request", "type": "any", "optional": true, "description": "The request sent by the calling script."},
           {"name": "sender", "$ref": "runtime.MessageSender" },
           {"name": "sendResponse", "type": "function", "description": "Function to call when you have a response. The argument should be any JSON-ifiable object, or undefined if there is no response." }
         ]
       }
     ]
   }
 ]