If you have control of both pages, then you could instead pass the requested title in the query string, for browsers that don't support postMessage().
And then in the other page:
window.open ("http://www.xxx.com?use%20this%20title" , "testpage" );
But if you don't have control of the other page and it is in another domain, there's really nothing you can do.
if ( location.search.length > 1 )
document.title = unescape(location.search.substring(1));