Bug 881092 - Allow decoding files we know we can't play, in the context of WebAudio. r=kinetik,rillian

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extra : rebase_source : ebad931aede2f8225404625e90a5f3228c4a31c0
Bug 881092 - Test that we can decode multichannel WAVE file only when using decodeAudioData. r=ehsan

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extra : rebase_source : fe1cb35be9a1197b01a6c90654abd825f28a67e5