Examine source code of Page Grid Ruler

{
  "manifest_version": 2,
  "version": "0.1.2",
  "name": "Page Grid Ruler",
  "description": "Draw a grid view, get page dimensions in pixel (px) unit and draw a rule",
  "permissions": [
    "storage",
    "contextMenus",
    "activeTab"
  ],
  "homepage_url": "https://webextension.org/listing/page-ruler.html",
  "icons": {
    "16": "/data/icons/16.png",
    "32": "/data/icons/32.png",
    "48": "/data/icons/48.png",
    "64": "/data/icons/64.png",
    "128": "/data/icons/128.png",
    "256": "/data/icons/256.png",
    "512": "/data/icons/512.png"
  },
  "background": {
    "scripts": [
      "/v2/chrome.action.js",
      "/v2/chrome.contextMenus.create.js",
      "/v2/chrome.runtime.js",
      "/v2/chrome.scripting.js",
      "worker.js"
    ]
  },
  "commands": {
    "_execute_browser_action": {}
  },
  "web_accessible_resources": [
    "/data/view/index.html"
  ],
  "browser_action": {},
  "browser_specific_settings": {
    "gecko": {
      "id": "{651ab369-b4f7-4185-9d73-11a1fe23c699}"
    }
  }
}