Examine source code of Flex Subnetting

{
  "manifest_version": 2,
  "name": "Flex Subnetting",
  "short_name": "Flexnet",
  "description": "IPv4 VLSM Subnet Calculator",
  "version": "1.3.2",
  "icons": {
    "16": "icon16.png",
    "48": "icon48.png",
    "128": "icon128.png"
  },
  "options_ui": {
    "page": "options.html"
  },
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": false
  },
  "browser_action": {
    "default_icon": "icon16.png"
  },
  "permissions": [
    "storage",
    "https://www.googleapis.com/"
  ],
  "default_locale": "en",
  "browser_specific_settings": {
    "gecko": {
      "id": "{3c035241-e804-49b3-bf5c-7ac0d7e775fa}"
    }
  }
}