I don't think we should allow media without audio tracks to autoplay because:
* It means play() before loaded metadata behaves differently than play()
called after loaded metadata.
* With the current impl we dispatch the "play" event and then the "pause"
event when we decide we should block, which may confuse some sites' controls.
* Delaying running the play() algorithm until we've loaded metadata would add
significant complexity, and may break sites.
* Chrome doesn't have this provision, meaning the complexity required to
support it will not result in much benefit to us.
MozReview-Commit-ID: FSVlDJAOisw
--HG--
extra : rebase_source : 93b1bcf8d8edbd6ca10ad918b40a87cd3cfbbf0b
