Examine source code of PlaybackRate for WhatsApp

{
  "manifest_version": 2,
  "name": "PlaybackRate for WhatsApp",
  "description": "Aumente ou diminua a velocidade dos audios do whatsapp",
  "version": "0.1",
  "icons": {
    "128": "128.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://web.whatsapp.com/"
      ],
      "css": [
        "styles.css"
      ],
      "js": [
        "content.js"
      ]
    }
  ]
}