Add pref media.block-play-until-visible from http://hg.mozilla.org/mozilla-central/rev/570ef52c51e9
Add pref media.block-play-until-visible from http://hg.mozilla.org/mozilla-central/rev/570ef52c51e9

nth10sd/funfuzz,MozillaSecurity/funfuzz,MozillaSecurity/funfuzz,MozillaSecurity/funfuzz,nth10sd/funfuzz,nth10sd/funfuzz