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}
f3850f6933