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

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extra : rebase_source : 1ede286a68f592038320ed5b82051bb9dd3cc0db
Bug 1087944 - Test for the promise returned by OfflineAudioContext.startRendering(). r=ehsan

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extra : rebase_source : 351f66c2cd7f2c269d7f2c05214d0f291488cf23