Javascript iframe在脚本向服务器应用新的用户代理之前将默认用户代理发送到服务器

Javascript:

<script type="text/javascript">
function setUserAgent(window, userAgent) {
    if (window.navigator.userAgent != userAgent) {
        var userAgentProp = { get: function () { return userAgent; } };
        try {
            Object.defineProperty(window.navigator, 'userAgent', userAgentProp);
        } catch (e) {
            window.navigator = Object.create(navigator, {
                userAgent: userAgentProp
            });
        }
    }
}
setUserAgent(window, '__MY AGENT__');
var vframe = document.getElementById('vframe');
if (vframe.contentDocument){
setUserAgent(vframe.contentDocument, '__MY AGENT__');;
}
else if (vframe.contentWindow){
setUserAgent(vframe.contentWindow, '__MY AGENT__');
}
document.write(window.navigator.userAgent); // __MY AGENT__
</script>

<iframe height="100px" width="100%" src="embed.php" id="vframe"></iframe>

document.write('<iframe height="100px" width="100%" src="embed.php" id="vframe"></iframe>');