PopupController = function () {
  this.button_ = document.getElementById('button');
  this.timeframe_ = document.getElementById('timeframe');
  this.addListeners_();
}
This class wraps the popup's form, and performs the proper clearing of data
based on the user's selections. It depends on the form containing a single
select element with an id of 'timeframe', and a single button with an id of
'button'. When you write actual code you should probably be a little more
accepting of variance, but this is just a sample app. :)

Most of this is boilerplate binding the controller to the UI. The bits that
specifically will be useful when using the BrowsingData API are contained in
`parseMilliseconds_`, `handleCallback_`, and `handleClick_`.

@constructor