Chrome extention example modify request headers of browser even 'Referer' etc.

background.js

// Headers you want to modify.
let modifyingHeaders = {
  "User-Agent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.100 Safari/537.36",
  "Origin": "http://example",
  "Referer": "http://example/foo/bar",
  "Cookie": 'key1="value1"; key2="value2";',
};

let requestFilter = {
  // urls: ["<all_urls>"]
  urls: ["*://example/*"]
};

let extraInfoSpec = [
  'requestHeaders',
  'blocking'
];

let handler = function(details) {
  let headers = details.requestHeaders;
  let blockingResponse = {};

  let header, headername, modifyingHeaderVal;
  // Modify target headers.
  for (let i = 0, len = headers.length; i < len; i++) {
    header = headers[i];
    headername = header.name;
    modifyingHeaderVal = modifyingHeaders[headername];
    if (modifyingHeaderVal) {
      header.value = modifyingHeaderVal
    }
  }

  // If header you want to change did not exist in headers[i], push that as new directly. 
  let exists;
  Object.keys(modifyingHeaders).forEach(function(k) {
    exists = headers[k];
    if (!exists) {
      headers.push({ name: k, value: modifyingHeaders[k] });
    }
  });

  blockingResponse.requestHeaders = headers;
  return blockingResponse;
};


// Execute header changing
chrome.webRequest
  .onBeforeSendHeaders
  .addListener(handler, requestFilter, extraInfoSpec);

manifest.json

{
  "name": "RequestHeaderChanger",
  "version": "0.0.1",
  "permissions": [
    "<all_urls>",
    "webRequest",
    "webRequestBlocking",
    "background",
    "activeTab"
  ],
  "background": {
    "scripts": ["background.js"]
    // "persistent": false
  },
  "browser_action": {
    "default_title": "popup",
    "default_icon": "icon.png",
    "default_popup": "popup.html"
  },
  "manifest_version": 2,
  "content_security_policy": "script-src 'self' https://ajax.googleapis.com; object-src 'self'"
}