Examine source code of Dawn Timer

{
  "name": "Dawn Timer",
  "version": "3.1.5",
  "description": "Times takes to send a message to a content script and back.",
  "content_scripts": [
    {
      "matches": [
        "http://*/*",
        "https://*/*"
      ],
      "js": [
        "page.js"
      ]
    }
  ],
  "browser_action": {
    "default_title": "Time to current page",
    "default_icon": "clock.png",
    "default_popup": "popup.html"
  },
  "manifest_version": 2
}