DO NOT MERGE: CaptivePortalLogin: set mixed content policy to compatibility.

This makes the policy for mixed HTTP/HTTPS content match Chrome.
This allows us to display login pages where unimportant
subresources are HTTP.

Bug: 32171569

(cherry picked from commit 62516637)

Change-Id: Ia82958192d978280715442e182c34f9da5c88c66