Use the MockVideoEncoder from the api target in
encoder_simulcast_proxy_unittest, rather than a custom MockEncoder.
This also prevents issue when new SetRates() is made pure virtual.
Bug: webrtc:10481
Change-Id: I72469803c00f7014eeac5b9321d1e0d716fa245d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/134211
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27774}
9e79e6b9b3