[web-audio-api] AudioBufferSourceNode.playbackRate valid range should be specified. (#202)

> Originally reported on W3C Bugzilla [ISSUE-21593](https://www.w3.org/Bugs/Public/show_bug.cgi?id=21593) Fri, 05 Apr 2013 15:34:25 GMT
> Reported by paul@paul.cx
> Assigned to 

The spec should mention what to do when the playbackRate is zero, and when it is negative.

fwiw, the |playbackRate| property for media element is specced as such:
- positive case is trivial
- zero pauses the element, without having |paused| == true
- negative plays the media backward.

WebKit treats negative and null value as illegal, and this is fine, but it should be specced.

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https://github.com/WebAudio/web-audio-api/issues/202