The mock is to be used in frame transformer unit tests.
Bug: webrtc:11380
Change-Id: Id3f6ec71712333232873d8de30e3c7392dc7f5e3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/174002
Commit-Queue: Marina Ciocea <marinaciocea@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31155}
