So that we don't have to be capable of creating one ourselves, which
requires a dependency on the audio codecs.
BUG=webrtc:8396
Change-Id: I5600da5e17f613b0e61a9fb0fbdb373fe42f855c
Reviewed-on: https://webrtc-review.googlesource.com/20220
Reviewed-by: Peter Thatcher <pthatcher@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20641}