Bug 857367 - Remove DecodePool::mShuttingDown, which is unnecessary. r=seth

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extra : rebase_source : a323f90de5d1616f50a6d9aa717c7b423f230c92
Bug 860149 - Mark ourselves as doing synchronous decoding when writing directly to decoders in AddSourceData. r=seth

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extra : rebase_source : 1d48d2ba8bc880bb9a52fd1f9f6a95849f12992a