postMessage example in Chrome Uber

uber.js

cr.define('uber', function() {
  ...
  function invokeMethodOnWindow(targetWindow, method, opt_params, opt_url) {
    var data = {method: method, params: opt_params};
    targetWindow.postMessage(data, opt_url ? opt_url : '*');
  }

  return {
    ...
    invokeMethodOnWindow: invokeMethodOnWindow
  };
});

... 
function updateNavigationControls() {
  var iframe = getSelectedIframe();
  uber.invokeMethodOnWindow(navFrame.firstChild.contentWindow, 'changeSelection', {pageId: iframe.id});
}