Add pref dom.audiochannel.mutedByDefault
Add pref dom.audiochannel.mutedByDefault

I noticed it in http://hg.mozilla.org/mozilla-central/rev/3745c3df8072, but it was originally added last month, in https://hg.mozilla.org/mozilla-central/rev/84dc903151c1.

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