Bug 1087944 - Implement the promise version of OfflineAudioContext.  r=ehsan,smaug

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extra : rebase_source : b5546f150ae8b2307e82dc8c6aae5bfbd5b5e670
Bug 1087944 - Test for the promise returned by OfflineAudioContext.startRendering(). r=ehsan

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extra : rebase_source : 134c1bb8684be1cefe9bd75efc9868eaf42a674e