[web-audio-api] Use Web IDL to define OfflineAudioContext's contructor (#236)

> Originally reported on W3C Bugzilla [ISSUE-20840](https://www.w3.org/Bugs/Public/show_bug.cgi?id=20840) Thu, 31 Jan 2013 19:44:12 GMT
> Reported by Ehsan Akhgari [:ehsan]
> Assigned to 

The proper Web IDL is:

[Constructor(unsigned long numberOfChannels, unsigned long length, float sampleRate)]
interface OfflineAudioContext : AudioContext {

    void startRendering();

    attribute OfflineRenderSuccessCallback oncomplete;

};

The comment behind the IDL in the spec can also go away.

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https://github.com/WebAudio/web-audio-api/issues/236