// manifest.json
// Replace content_scripts with _content_scripts to see that
// Changing the manifest file does not update the extension.
//  (or simply change the version number)
{
  "name": "Reload test",
  "version": "1",
  "content_scripts": [{
    "matches": ["<all_urls>"],
    "js": ["script.js"]
  }],
  "background": {"scripts":["bg.js"]}
}

// script.js
alert('Test');

// bg.js
function reloadOnChange(url, checkIntervalMS) {
    if (!window.__watchedFiles) {
        window.__watchedFiles = {};
    }

    (function() {
        var self = arguments.callee;
        var xhr = new XMLHttpRequest();

        xhr.onreadystatechange = function() {
            if (xhr.readyState == 4) {
                if (__watchedFiles[url] &&
                    __watchedFiles[url] != xhr.responseText) {
                    window.location.reload();
                } else {
                    __watchedFiles[url] = xhr.responseText
                    window.setTimeout(self, checkIntervalMS || 1000);
                }
            }
        };

        xhr.open("GET", url, true);
        xhr.send();
    })();
}
console.log('loaded')
reloadOnChange(chrome.extension.getURL('/manifest.json'));