Bug#821759: Iceweasel does not warn about website asking to store data for offline use

[Alef Farah]

Package: iceweasel
Version: 44.0.2-1

There is an option in iceweasel, under "about:preferences#advanced",
which says "Tell me when a website asks to store data for offline use",
which consists of a checkbox. It is expected that, when the box is
checked, a dialog would appear when a website wishes to store data for offline
use. What seems to happen is nothing: No dialog and no warning.

Steps to reproduce: Check the box as aforementioned, then visit a
website that stores data for offline use. Some examples that are present
in my browser:

http://www.thesun.co.uk
https://web.whatsapp.com
https://web.telegram.org

I am using Debian stretch/sid.