DevTools: provide the debugger detach reason in chrome.debugger.onDetach extension API.

chrome/common/extensions/api/debugger.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": "debugger",
     "types": [
       {
         "id": "Debuggee",
@@ skipping 115 matching lines @@
       },
       {
         "name": "onDetach",
         "type": "function",
         "description": "Fired when browser terminates debugging session for the tab. This happens when either the tab is being closed or Chrome DevTools is being invoked for the attached tab.",
         "parameters": [
           {
             "$ref": "Debuggee",
             "name": "source",
             "description": "The debuggee that was detached."
+          },
+          {
+            "type": "string",
+            "name": "reason",
+            "description": "Connection termination reason.",
+            "enum": [ "target_closed", "canceled_by_user", "replaced_with_devtools" ]
           }
         ]
       }
     ]
   }
 ]