This makes it possible to test custom network controllers without requiring update to test framework. Also updating BBR performance test to use this feature. Bug: webrtc:9510 Change-Id: I0446de0403fe9d1f6dc3710c1d114887a6c359c5 Reviewed-on: https://webrtc-review.googlesource.com/c/114640 Commit-Queue: Sebastian Jansson <srte@webrtc.org> Reviewed-by: Christoffer Rodbro <crodbro@webrtc.org> Cr-Commit-Position: refs/heads/master@{#26046}