Browser saving password with AJAX form

I'm making a web-project on PHP and it has a login form. For some reasons, the form is sending by AJAX request. So, browser does not prompt for saving login/password pair. I've googled a lot, so please don't send me there. Everywhere there is one solution.

<iframe src="about:blank" name="login_frame" style="display: none;"></iframe>
<form target="login_frame" action="about:blank" method="post" autocomplete="on" >
    <input type="text" name="username"/>
    <input type="password" name="password"/>
    <input type="submit" value="submit"/>
</form>

It works good for IE 11 and Firefox 37, but doesn't work for Chrome 44 and Opera 31. Could you please tell me how to force Chrome and Opera to prompt a 'save password' dialog?