To make VideoCodec::scalability_mode the only option to set and
change the scalability structure, for easier maintainability.
Bug: webrtc:11404
Change-Id: I6570e9a93ddf2897ff7584c5d20a246346e853e2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/192361
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33056}