Make back button work in CaptivePortalLogin app like a web browser.

Users will be using the app to navigate web pages like they are used to in
web browsers, so the back button should act similarly.  There also isn't
an in-app back button so this adds essentially necessary functionality.

bug:16397616
Change-Id: Idd73d4a048d79e56bf981a8f477336f00eaafc3b