Examine source code of Click-2-Call

{
  "name": "Click to Call",
  "description": "Enable click-to-call functionality.",
  "version": "1.6",
  "manifest_version": 2,
  "icons": {
    "16": "images/icon16.png",
    "48": "images/icon48.png",
    "128": "images/icon128.png"
  },
  "browser_action": {
    "browser_style": false,
    "default_icon": {
      "16": "images/icon16.png",
      "48": "images/icon48.png"
    },
    "default_title": "Click to Call",
    "default_popup": "popup.html"
  },
  "background": {
    "scripts": [
      "js/background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "js/content.js"
      ],
      "run_at": "document_idle"
    }
  ],
  "permissions": [
    "tabs",
    "storage"
  ]
}