[whatwg] hasAudio JS only property for video element

[Biju]

from https://bugzilla.mozilla.org/show_bug.cgi?id=480376

> It's not too uncommon for videos to have no audio track. It would be really
> nice if the video controls could indicate this, so that users know why there's
> no sound ("is something broken? is my volume too low? wtf?").
>
> Unfortunately this info isn't available through the media element API, so this
> would need to be added to the HTML5 spec. The simplest way to expose this would
> be as |readonly boolean hasAudio|. Is the media backend capable of determining
> this this?

we need a hasAudio JS only property for video element