So that we don't have to be capable of creating one ourselves, which requires a dependency on the audio decoders. In the process, remove the default values for the VoEBase::Init() arguments, since there isn't a sensible default value for the audio decoder factory anymore. BUG=webrtc:8396 Change-Id: Idb433efa49e1a68e8206d369d27b3c255185777a Reviewed-on: https://webrtc-review.googlesource.com/18200 Commit-Queue: Karl Wiberg <kwiberg@webrtc.org> Reviewed-by: Henrik Andreassson <henrika@webrtc.org> Cr-Commit-Position: refs/heads/master@{#20552}