Examine source code of Responsive dev

{
  "manifest_version": 2,
  "name": "Responsive dev",
  "version": "0.1",
  "description": "Easy development tool that shows your website on multiple screens",
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "index.js"
      ]
    }
  ],
  "web_accessible_resources": [
    "background.html",
    "test.html",
    "device.min.css"
  ]
}