Fixes a bug in content.js when showing the pop up a 2nd time

Fixes a bug in content.js when showing the pop up a 2nd time on some sites where there isn't a password field (http://www.favbrowser.com/mozilla-ditches-plans-to-display-ads-in-a-new-tab/), the pop up shows the "Fill Field" button anyway.  The chrome run-time system is keeping the response state around somewhere so I'm explicitly returning a "false" message instead of implying a false response.

Also has 2 non-functional style changes.