<webview>: Make contentWindow available prior to attachment (on display:none).

extensions/common/api/guest_view_internal.json

 // Copyright 2014 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": "guestViewInternal",
     "compiler_options": {
       "implemented_in": "extensions/browser/api/guest_view/guest_view_internal_api.h"
     },
@@ skipping 48 matching lines @@
           {
             "type": "object",
             "name": "createParams",
             "additionalProperties": {"type": "any"}
           },
           {
             "type": "function",
             "name": "callback",
             "parameters": [
               {
-                "type": "integer",
-                "name": "instanceId"
+                "type": "object",
+                "name": "returnParams",
+                "additionalProperties": {"type": "any"}
               }
             ]
           }
         ]
       },
       {
         "name": "destroyGuest",
         "type": "function",
         "parameters": [
           {
@@ skipping 25 matching lines @@
             "type": "function",
             "name": "callback",
             "parameters": [],
             "optional": true
           }
         ]
       }
     ]
   }
 ]