Prevent duplicate concurrent installs of the same extension

chrome/common/extensions/api/webstore_private.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":"webstorePrivate",
     "description": "none",
     "functions": [
       {
@@ skipping 289 matching lines @@
             "description": "The extension id of the app to launch."
           },
           {
             "name": "callback",
             "type": "function",
             "optional": true,
             "parameters": [
               {
                 "name": "result",
                 "type": "string",
-                "enum": ["success", "user_gesture_required", "unknown_error", "feature_disabled", "unsupported_extension_type", "missing_dependencies", "install_error", "user_cancelled", "invalid_id", "blacklisted", "blocked_by_policy", "install_in_progress"],
+                "enum": ["success", "user_gesture_required", "unknown_error", "feature_disabled", "unsupported_extension_type", "missing_dependencies", "install_error", "user_cancelled", "invalid_id", "blacklisted", "blocked_by_policy", "install_in_progress", "launch_in_progress"],
                 "description": "Whether an attempt to launch an app succeeded, or the reason for failure."
               }
             ]
           }
         ]
       }
     ]
   }
 ]