manifest with unexpected property

{
  "manifest_version": 2,
  "name": "Broccoli",
  "version": "0.1.0",
  "description": "Broccoli tests",
  "browser_action": {
    "default_popup": "./popup.html",
    "default_icon": {
      "16": "img/broccoli16.png",
      "48": "img/broccoli48.png",
      "128": "img/broccoli128.png"
    },
    "default_title": "Broccoli"
  },
  "content_security_policy": "script-src 'self' https://www.gstatic.com/ https://*.firebaseio.com https://www.googleapis.com; object-src 'self'",
  "content_scripts": [
    {
      "css": [
        "extra.css",
        "all.css",
        "bootstrap.min.css"
      ],
      "js": [
        "firebase.js",
        "jquery.min.js",
        "content.js",
        "popper.min.js",
        "bootstrap.min.js"
      ],
      "matches": [
        "https://dlab.epfl.ch/*",
        "https://*.wikipedia.org/*"
      ],
      "permissions": [
        "storage",
        "activeTab"
      ]
    }
  ],
  "background": {
    "scripts": [
      "popup.js"
    ],
    "persistent": false
  }
}