Examine source code of Mobrem

{
  "manifest_version": 2,
  "name": "Mobrem",
  "description": "Mobile browser emulator mainly for testing the responsiveness of web pages on various mobile screen sizes",
  "version": "1.5.3",
  "icons": {
    "16": "images/icon_16.png",
    "48": "images/icon_48.png",
    "128": "images/icon_128.png"
  },
  "browser_action": {
    "default_title": "Mobrem for responsive design",
    "default_icon": "images/icon_20.png",
    "default_popup": "popup.html"
  },
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*"
      ],
      "js": [
        "emulator.js"
      ],
      "css": [
        "emulator.css"
      ]
    }
  ],
  "background": {
    "scripts": [
      "background_script.js"
    ],
    "persistent": true
  },
  "permissions": [
    "activeTab",
    "storage"
  ]
}