Chrome User-Agent String Modification

background-toggle.js

var listenerIsActive = true,
  requestFilter = {
    urls: [ "<all_urls>" ]
  },
  extraInfoSpec = ['requestHeaders','blocking'],
  handler = function( details ) {

    var headers = details.requestHeaders,
      blockingResponse = {};
    
    if( !listenerIsActive ) {
      // If you take this approach, just make sure you return
      // the headers, or else your "disabled" listener will
      // delete all headers from future requests!
      return {requestHeaders: details.requestHeaders};
    }

    // Your code here...
  };

chrome.webRequest.onBeforeSendHeaders.addListener( handler, requestFilter, extraInfoSpec );

background.js

  // The 'reqestFilter' parameter allows you to only listen for
  // certain requests. Chrome 17 requires that, at the very least,
  // it defines the URLs you wish to subscribe to. In the general
  // case, we want to subscribe to all URL's, so we'll explicitly
  // declare this requirement.
var requestFilter = {
    urls: [ "<all_urls>" ]
  },
  // The 'extraInfoSpec' parameter modifies how Chrome calls your
  // listener function. 'requestHeaders' ensures that the 'details'
  // object has a key called 'requestHeaders' containing the headers,
  // and 'blocking' ensures that the object your function returns is
  // used to overwrite the headers
  extraInfoSpec = ['requestHeaders','blocking'],
  // Chrome will call your listener function in response to every
  // HTTP request
  handler = function( details ) {

    var headers = details.requestHeaders,
      blockingResponse = {};

    // Each header parameter is stored in an array. Since Chrome
    // makes no guarantee about the contents/order of this array,
    // you'll have to iterate through it to find for the
    // 'User-Agent' element
    for( var i = 0, l = headers.length; i < l; ++i ) {
      if( headers[i].name == 'User-Agent' ) {
        headers[i].value = '>>> Your new user agent string here <<<';
        break;
      }
      // If you want to modify other headers, this is the place to
      // do it. Either remove the 'break;' statement and add in more
      // conditionals or use a 'switch' statement on 'headers[i].name'
    }

    blockingResponse.requestHeaders = headers;
    return blockingResponse;
  };

chrome.webRequest.onBeforeSendHeaders.addListener( handler, requestFilter, extraInfoSpec );

requestHeaders.json

[
	{"name":"Pragma","value":"no-cache"},
	{"name":"User-Agent","value":"Mozilla/5.0 (Windows NT 5.1) AppleWebKit/535.1 (KHTML, like Gecko) Chrome/14.0.835.15 Safari/535.1"},
	{"name":"Accept","value":"text/css,*/*;q=0.1"},
	{"name":"Cache-Control","value":"no-cache"},
	{"name":"Referer","value":"http://mikepennisi.com/"},
	{"name":"Accept-Encoding","value":"gzip,deflate,sdch"},
	{"name":"Accept-Language","value":"en-US,en;q=0.8"},
	{"name":"Accept-Charset","value":"ISO-8859-1,utf-8;q=0.7,*;q=0.3"},
	{"name":"Cookie","value":"__utma=138710319.518446708.1311977381.1311977381.1311977381.1; __utmz=138710319.1311977381.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); ci_session=6YkLJxXZRFbZ5DLlpFCL8KG3cnggPjSvEtP7VCNRTv4JLphOrHGUqPX3pfOuURl0cfbMb9Gu4%2FcNnsxZyiTNxH%2B0hXZ4pElZush0NJNHHD7ReOV2hoHWRaDX1ikw4vWamoyB8WvLftxotbKX4ibITIwqBlQ0mqq%2BcIQRn5uW0ueFgwzt43b5Q6KBGYJ2RMXZrrT%2FgB0ViV8dofvXNmQ8qFueXOlotnnWx1KcTWyyiWDOIia2%2Bm5ZuknlbmvFMk5czOMf5y70S4oaK7iup7gAlaPSVD9%2FZE%2BzslzuCvvyiXM%3D"}
]