Define MockTransformableVideoFrame in api/test to be use in Blink
tests.
Bug: chromium:1069295
Change-Id: I960d8228eddbdf8263b731814a00fd02d0a6c54e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/175136
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Marina Ciocea <marinaciocea@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31272}
