Examine source code of localhost3000

{
  "description": "Open http://localhost:3000",
  "manifest_version": 2,
  "name": "localhost3000",
  "version": "1.0",
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "browser_action": {
    "default_icon": {
      "16": "icons/rails.png",
      "32": "icons/rails.png"
    }
  }
}