Examine source code of ClockZilla

{
  "manifest_version": 2,
  "name": "ClockZilla",
  "version": "1.2",
  "description": "Add a funny clock in your browser",
  "icons": {
    "64": "icons/2605807-64.png"
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "ClockZilla.js"
      ]
    }
  ]
}