So that we don't have to be capable of creating one ourselves, which requires a dependency on the audio decoders. BUG=webrtc:5801, webrtc:8396 Change-Id: I80749ec3b86cba73994307046d05964f59167d44 Reviewed-on: https://webrtc-review.googlesource.com/18440 Commit-Queue: Karl Wiberg <kwiberg@webrtc.org> Reviewed-by: Oskar Sundbom <ossu@webrtc.org> Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org> Cr-Commit-Position: refs/heads/master@{#22774}