Examine source code of sessionSave

{
  "manifest_version": 2,
  "name": "sessionSave",
  "version": "1.1",
  "description": "Saves all closed windows temporarily.",
  "icons": {
    "48": "icons/border-48.png"
  },
  "permissions": [
    "sessions",
    "storage",
    "tabs"
  ],
  "browser_action": {
    "default_icon": "icons/tooltip-32.png",
    "default_title": "sessionSave"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*/*"
      ],
      "js": [
        "popup/sessionSave.js"
      ]
    }
  ],
  "background": {
    "scripts": [
      "sessionSaveBackground.js"
    ]
  }
}