Examine source code of Music Progress

{
  "manifest_version": 2,
  "name": "Music Progress",
  "version": "1.0",
  "description": "Plays a loading-loop whose pitch rises with page load progress; plays a short finish when load completes.",
  "icons": {},
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": true
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "music-progress.js"
      ],
      "run_at": "document_start"
    }
  ],
  "permissions": [
    "webNavigation",
    "tabs",
    "activeTab",
    "storage"
  ],
  "web_accessible_resources": [],
  "browser_specific_settings": {
    "gecko": {
      "id": "{309f46de-aefd-4451-a1cb-a9c4265a2a17}"
    }
  }
}