Count the length of the character

manifest.json

{
  "update_url": "https://clients2.google.com/service/update2/crx",
  "manifest_version": 2,
  "default_locale": "en",
  "name": "__MSG_extName__",
  "version": "0.1",
  "description": "__MSG_extDescription__",
  "icons": {
    "128": "icon_128.png"
  },
  "browser_action": {
    "default_popup": "html/popup.html",
    "default_icon": "icon_128.png"
  },
  "background": {
    "scripts": [
      "js/popup.js"
    ],
    "persistent": false
  }
}