Bug 806754. Make MediaDecoders detect when a MediaStream has been destroyed because its DOM wrapper has been collected, and remove the no-longer-relevant MediaStream from the output list. r=cpearce

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extra : rebase_source : a7316907f453330c0c23eb7a11afb3c2cd5a1e9c
Bug 806754. Part 2: Fix bogus assertions. r=cpearce
SendStreamAudio gets called by SendStreamData which can be called
on the state machine thread since bug 794426 was fixed.
At the same time PlayFromAudioQueuec can no longer guarantee that
mAudioCaptured is false. It could be true and we're waiting for
the audio thread to shut down. We can just remove that assertion;
the logic in SendStreamData guarantees that we don't try to pass
audio to MediaStreams until the audio thread has actually stopped.

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extra : rebase_source : 431b4af63710ef13a57f7560aeec553b8aae6139