Examine source code of FPS meter

{
  "manifest_version": 2,
  "name": "FPS meter",
  "version": "1.1",
  "description": "Show fps graph",
  "icons": {
    "48": "icons/baseline_equalizer_black_48dp.png"
  },
  "permissions": [
    "activeTab"
  ],
  "browser_action": {
    "default_icon": "icons/baseline_equalizer_black_48dp.png",
    "default_title": "FPS meter"
  },
  "background": {
    "scripts": [
      "browser-polyfill.min.js",
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "browser-polyfill.min.js",
        "main.js"
      ],
      "css": [
        "main.css"
      ]
    }
  ]
}