So that we don't have to be capable of creating one ourselves, which
requires a dependency on the audio decoders.
BUG=webrtc:6000, webrtc:8396
Change-Id: Ibb6b3f36f14b956c55d4edc934d101cb855b272d
Reviewed-on: https://webrtc-review.googlesource.com/18420
Reviewed-by: Fredrik Solenberg <solenberg@webrtc.org>
Reviewed-by: Oskar Sundbom <ossu@webrtc.org>
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20622}