Only use replaced with FakePeriodicVideoTrackSource.
Bug: webrtc:6353
Change-Id: Iee38b98a5242a292a848738bde05de18d96de7f4
Reviewed-on: https://webrtc-review.googlesource.com/79441
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23441}