Unrecognized manifest key 'gcm_sender_id' ,Chrome extension linked to fcm

am trying to link a chrome extension to firebase cloud messaging, but am getting Unrecognized manifest key 'gcm_sender_id' error when i load the extension image of the error, here is my manifest file:

   {

  "manifest_version": 2,
  "name": "iQU",
  "description": "iQU",
  "version": "1.0",
  "browser_action": {
    "default_icon": "icon.png",
    "default_popup": "index.html"
  },
  "permissions": [],
  "content_security_policy": "script-src 'self' 'unsafe-eval' https://cdn.firebase.com https://*.firebaseio.com https://*.firebaseio.com; object-src 'self'; ",
  "web_accessible_resources": [
    "assets/css/*",
    "assets/js/*",
    "assets/fonts/*"
  ],
  "gcm_sender_id": "103953800507"
}

any help please?