fx28: add an option |disableMCB|

* Persist "disable protection" option for Mixed Content Blocker in child tabs
  https://bugzilla.mozilla.org/show_bug.cgi?id=906190
fx28: remove an option |isUTF8|

* Remove FIXUP_FLAG_USE_UTF8
  https://bugzilla.mozilla.org/show_bug.cgi?id=945627