eventPage.js

sites_url = "https://raw.githubusercontent.com/babolivier/collateral-freedom/master/sites.json"
sites = {}

function getSitesAndMirrors() {
    $.getJSON(sites_url).done(function(data) { chrome.storage.local.set({"sites": data}) })
}

function getCurrentTabUrl(callback) {
  // Query filter to be passed to chrome.tabs.query - see
  // https://developer.chrome.com/extensions/tabs#method-query
  var queryInfo = {
    active: true,
    currentWindow: true
  };

  chrome.tabs.query(queryInfo, function(tabs) {
    // chrome.tabs.query invokes the callback with a list of tabs that match the
    // query. When the popup is opened, there is certainly a window and at least
    // one tab, so we can safely assume that |tabs| is a non-empty array.
    // A window can only have one active tab at a time, so the array consists of
    // exactly one tab.
    var tab = tabs[0];

    // A tab is a plain object that provides information about the tab.
    // See https://developer.chrome.com/extensions/tabs#type-Tab
    var url = tab.url;

    // tab.url is only available if the "activeTab" permission is declared.
    // If you want to see the URL of other tabs (e.g. after removing active:true
    // from |queryInfo|), then the "tabs" permission is required to see their
    // "url" properties.
    console.assert(typeof url == 'string', 'tab.url should be a string');

    callback(url);
  });

  // Most methods of the Chrome extension APIs are asynchronous. This means that
  // you CANNOT do something like this:
  //
  // var url;
  // chrome.tabs.query(queryInfo, function(tabs) {
  //   url = tabs[0].url;
  // });
  // alert(url); // Shows "undefined", because chrome.tabs.query is async.
}

function updateTab() {
    getCurrentTabUrl(function(url) {
        chrome.storage.local.get("sites", function(sites){
            sites = sites.sites
            if(url.match(/:\/\/([^\/]+)\//)[1] in sites) {
                chrome.browserAction.setIcon({path: 'icon-red.png'})
            }
            else {
                chrome.browserAction.setIcon({path: 'icon.png'})
            }
        })
    })
}

chrome.runtime.onStartup.addListener(getSitesAndMirrors)
chrome.runtime.onInstalled.addListener(getSitesAndMirrors)
chrome.tabs.onActivated.addListener(updateTab)
chrome.tabs.onUpdated.addListener(updateTab)