background.js

// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

'use strict';

chrome.runtime.onInstalled.addListener(function() {
  chrome.storage.sync.set({number: 1}, function() {
    console.log('The number is set to 1.');
  });
});

function updateIcon() {
  chrome.storage.sync.get('number', function(data) {
    var current = data.number;
    chrome.browserAction.setIcon({path: 'icon' + current + '.png'});
    current++;
    if (current > 5)
      current = 1;
    chrome.storage.sync.set({number: current}, function() {
      console.log('The number is set to ' + current);
    });
  });
};

chrome.browserAction.onClicked.addListener(updateIcon);
updateIcon();