Bug 856602 - RasterImage::DecodeJob should not reschedule itself if it can't make progress. r=tn

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extra : rebase_source : 0ef4698a0104bab8961d84e933d8eca28eb77111
Bug 856602 - Uplift to Aurora. a=lsblakk
Bug 857367 - Make it safe to call RasterImage::DecodePool::RequestDecode off the main thread. r=tn
Bug 857367 - Uplift to Aurora. a=lsblakk