Merge pull request #15744 from mikehenrty/bug-959800-mozbrowser

Bug 959800 - [Offline] mozbrowser iframes don't display correct offline ...
(cherry picked from commit f1928155eb29b6b1766ba6ca8380221a4f670d60)

Conflicts:
	apps/system/js/net_error.js
	build/jshint/xfail.list
Merge pull request #15900 from mikehenrty/bug-959800-uplift

Merge pull request #15744 from mikehenrty/bug-959800-mozbrowser, r=auswer