This simplifies creations of frame generator capturers in a reusable
way. It's modelled on the scenario VideoSendStreamConfig,
Bug: webrtc:10839
Change-Id: Ibe0709cd94521f78c6267eece533b048607d0994
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/147272
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28722}
