Examine source code of Timer for youtube

{
  "manifest_version": 2,
  "name": "Youtube timer",
  "version": "1.1",
  "description": "Counts how much time you spent on youtube.com",
  "content_scripts": [
    {
      "matches": [
        "*://*.youtube.com/*"
      ],
      "js": [
        "content_scripts/counter.js"
      ]
    }
  ],
  "permissions": [
    "storage",
    "tabs"
  ],
  "browser_action": {
    "default_icon": "icons/clock.png",
    "default_title": "Youtube timer",
    "default_popup": "popup/index.html"
  },
  "browser_specific_settings": {
    "gecko": {
      "id": "adomasdauda@github.com",
      "strict_min_version": "57.0a1"
    }
  }
}