Allow chrome.debugger API to attach to extension background pages.

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",
         "type": "object",
-        "description": "Debuggee identifier.",
+        "description": "Debuggee identifier. Either tabId or extensionId must be specified",
         "properties": {
-          "tabId": { "type": "integer", "description": "The id of the tab which you intend to debug." }
+          "tabId": { "type": "integer", "optional": true, "description": "The id of the tab which you intend to debug." },
+          "extensionId": { "type": "string", "optional": true, "description": "The id of the extension which you intend to debug. Attaching to an extension background page is only possible when 'enable-silent-debugging' flag is enabled on the target browser." }
         }
       }
     ],
     "functions": [
       {
         "name": "attach",
         "type": "function",
         "description": "Attaches debugger to the given target.",
         "parameters": [
           {
@@ skipping 113 matching lines @@
             "type": "string",
             "name": "reason",
             "description": "Connection termination reason.",
             "enum": [ "target_closed", "canceled_by_user", "replaced_with_devtools" ]
           }
         ]
       }
     ]
   }
 ]