Examine source code of Headers Middleman

{
  "name": "Headers Middleman",
  "description": "Add, remove, or modify HTTP Headers for any request based on a regular expression",
  "manifest_version": 2,
  "version": "1.4.2resigned1",
  "icons": {
    "128": "images/icon/icon-128.png"
  },
  "applications": {
    "gecko": {
      "id": "headersmiddleman@dingoeatingfuzz",
      "strict_min_version": "53.0"
    }
  },
  "homepage_url": "https://github.com/DingoEatingFuzz/chrome-headers-middleman",
  "permissions": [
    "webRequest",
    "webRequestBlocking",
    "<all_urls>",
    "storage"
  ],
  "options_ui": {
    "open_in_tab": true,
    "page": "options.html"
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  }
}