Examine source code of Reurl.cc

{
  "manifest_version": 2,
  "name": "Reurl.cc",
  "description": "Extension to shorten URL",
  "version": "2.2.4",
  "permissions": [
    "cookies",
    "activeTab",
    "https://*.reurl.cc/"
  ],
  "browser_action": {
    "default_title": "Reurl.cc",
    "default_icon": "img/icon16.png",
    "default_popup": "popup.html"
  },
  "icons": {
    "16": "img/icon16.png",
    "48": "img/icon48.png",
    "128": "img/icon128.png"
  }
}